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Evaluation of California’s Graduated
Driver Licensing Program
May 2003
Authors: Scott V. Masten and Robert A. Hagge
Research and Development Branch
© California Department of Motor Vehicles, 2003 Licensing Operations Division
RSS-03-205
STATE OF CALIFORNIA
DEPARTMENT OF MOTOR VEHICLES
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
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Final Report
Scott V. Masten and Robert A. Hagge
62
Evaluation of California’s Graduated Driver Licensing Program
California’s 1998 graduated driver licensing program was implemented to reduce the high crash risk o
f
teenage drivers. Monthly per capita crash rates for 15-to-17-year-olds were analyzed using time serie
s
analysis. No overall reduction in total crashes or fatal/injury crashes was found immediately followin
g
program implementation or beginning 6 months later. The 12-month nighttime restriction was associate
d
with significant sudden-permanent reductions of 0.44% in total crashes and marginally significant 0.45%
in nighttime fatal/injury crashes. The 6-month passenger restriction was associated with reductions o
f
2.52% and 6.43% in total and fatal/injury teen passenger crashes, respectively. The fact that no overal
l
reductions in crashes, and only small reductions in crashes associated with the restrictions, were found is
not surprising given findings that teens and parents were either already practicing program requirements
prior to implementation, or not fully complying with the program require
ments afterwards. The finding
s
provide support for passenger and nighttime restrictions.
California; GDL; graduated driver licensing; nighttime restriction; passenger
restriction
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
i
PREFACE
This report presents the results of an evaluation of the traffic safety impact of
California’s Graduated Driver Licensing Program for drivers younger than 18 years of
age (CVC §12814.6). The purpose of the evaluation is to provide statistical information
useful to traffic safety researchers and driver licensing program administrators in
determining the effectiveness of the teen licensing program and its major components
in reducing crashes involving young drivers. This project is part of the California Traffic
Safety Program and was made possible through the support of the California Office of
Traffic Safety, State of California, and the National Highway Traffic Safety
Administration. The report was prepared by the Research and Development Branch of
the California Department of Motor Vehicles under the administration of Cliff
Helander, Chief. The opinions, findings, and conclusions expressed in this publication
are those of the authors and not necessarily those of the State of California or the
National Highway Traffic Safety Administration.
ACKNOWLEDGMENTS
This study was conducted under the supervision of Robert Hagge, Research Manager II,
who also contributed the design of the study, supervised all of the statistical analyses,
and edited the final report. Emilie Mitchell, former Research Analyst II, wrote much of
the computer code used to create the crash series, completed initial time series analyses,
and wrote detailed instructions that eased the transition for the subsequent project
leader. Eric Chapman, Research Analyst II, helped create the tables and helped in the
analysis and interpretation of the time series results. Debbie McKenzie, Associate
Governmental Program Analyst, helped create the figures and format the final report.
EXECUTIVE SUMMARY
Introduction
Teenage drivers have a much higher crash risk than do older drivers due to their
fundamental lack of driving skill, inexperience at driving, tendency towards increased
risk-taking, immaturity, inaccurate risk perception, and overestimation of driving skills
(Janke, Masten, McKenzie, Gebers, & Kelsey, 2003). States have tried to mitigate the
increased crash risk of teenagers by implementing modified driver licensing programs
for teenagers that focus on improving their skills and reducing their exposure to those
situations in which they are at the highest risk, such as driving at night or with young
passengers. The modified licensing systems for teenagers usually include several stages
leading to an unrestricted license. The licensing stages for teenagers typically include a
supervised practice period, license restrictions, and accelerated post-licensing control
actions that do not apply to adults. This report presents results of an evaluation of the
safety impact of several enhancements made in July 1998 to improve the effectiveness of
California’s licensing program for drivers under age 18.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
ii
California’s first teen licensing program (called the provisional licensing program),
implemented in October 1983, included all of the following components for license
applicants under age 18:
• A mandatory 1-month instruction permit period allowing driving only when
supervised by a parent/guardian, spouse, or licensed adult 25 years of age or older.
• A parent/teen driver-practice guide that contains structured driving exercises that
the teen must master before taking a drive test.
• A distinctive looking driver license, allowing easy identification of the driver as a
provisional licensee.
• A 1-week wait after failing the written knowledge test and 2-week wait after failing
the behind-the-wheel drive test before retesting.
• Parent certification that the teen successfully completed the exercises in the
parent/teen guide and is skilled enough to pass the DMV drive test.
• An accelerated post-licensing control action program in which teens receive a
warning letter after their first traffic violation or responsible crash, a 1-month
restriction allowing only supervised driving after their second violation or at-fault
crash in a 12-month period, a 6-month license suspension and 1-year probation after
a third offense in 12 months, and extended license suspension or possible revocation
after a fourth offense, violation of probation, failure to appear in court, or failure to
pay a fine.
Hagge and Marsh (1988) evaluated the California provisional licensing program using
time series analysis and also an assessment of individual driver records. They found
that the program as a whole was associated with 5.3% lower per capita crash rates for
15- to-17-year-olds and 23% lower violation rates for 16-year-old licensed drivers.
California Vehicle Code Section 12814.6 added enhancements to the teen driver license
program starting in July 1998. This program is called the graduated driver licensing
(GDL) program. In addition to having to pass the vision, written, and drive tests, the
California graduated licensing program evaluated in this report includes all of the
components of the original provisional licensing program identified above plus:
• A minimum 6-month instruction permit period.
• Parent/guardian certification that the teen driver completed a minimum of 50 hours
of behind-the-wheel practice (10 hours of which must be at night) supervised by a
licensed parent/guardian, spouse, or adult 25 years of age or older, or a certified
driving instructor.
• A 12-month restriction from driving between 12:00 a.m. and 5:00 a.m., unless
supervised as defined above. Exceptions are granted for medical or family necessity,
school activities, and employment needs, with a note signed by the proper authority
such as a parent or principal and specifying the ending date for the exception.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
iii
• A 6-month restriction from driving with passengers under the age of 20, unless
supervised as defined above. Exceptions are allowed under the same circumstances
indicated above.
Method
Monthly statewide per capita crash rates for January 1994 to December 2001 were
analyzed using Autoregressive Integrated Moving Average (ARIMA) intervention time
series analysis to determine whether implementing the GDL enhancements in July 1998
changed the rate of crashes involving 15-to-17-year-old drivers, and in some cases the
rates of crashes involving 16-year-old and 18-19-year-old drivers. The crash rates for
adult drivers aged 24 to 55 were used as a control series in some of the analyses to
account for history-related factors that would have affected crashes for both age groups.
The following criterion crash series were created and analyzed in this evaluation:
1. Total crashes
2. Fatal/injury crashes
3. Proportion of total crashes occurring during 12:00-5:00 a.m.
4. Proportion of fatal/injury crashes occurring during 12:00-5:00 a.m.
5. Proportion of total crashes involving passengers under age 20
6. Proportion of fatal/injury crashes involving passengers under age 20
7. Total crashes involving 16-year-olds
8. Fatal/injury crashes involving 16-year-olds
9. Total crashes involving 18-19-year-olds
10. Fatal/injury crashes involving 18-19-year-olds
The first two series were analyzed to evaluate the impact of GDL as a whole. The
analyses of crashes in which a 16-year-old was the youngest involved driver are
conceptually less biased for purposes of evaluating the impact of the GDL
enhancements, because of a shorter transition time period for all drivers in this age
group to be completely under the new GDL program requirements. The four series
involving proportions of crashes during the restricted time period and involving
passengers less than 20 years of age were used to evaluate the impact of the nighttime
restriction and passenger restriction components of GDL, respectively. The analyses of
18-19-year-old drivers in crashes were conducted to determine if the program had any
positive or negative effects on this age group. Two additional crash series not listed
above were also analyzed. These consisted of crash involvements for which a single
crash incident was typically assigned multiple times (one “crash” count assigned to
each driver involved).
Results
This study analyzed several different crash types and age-groups, various intervention
models, and flexible intervention start points to determine whether the enhancements
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
iv
made to the California teen licensing program in July 1998 resulted in crash reductions
for teen drivers. The results are summarized below:
• No overall reduction in total crashes or fatal/injury crashes was found immediately
following program implementation or beginning 6 months later. This outcome was
the same even when transition components were added to the models to adjust for
the influence of the influx of teen licensees before the implementation date, when
the adult series was included as a control variable, when only 16-year-old driver
crashes were analyzed, and when the rates were calculated as crash involvements
rather than being based on the youngest involved driver. However the program
was found to be associated with a 19.45% gradual-permanent increase in total
crashes for 18-19-year-olds 6 months after the program was implemented (about
9,464 additional crashes per year). No significant effect was found in the 18-19-year-
olds fatal/injury crashes.
• The 12-month nighttime restriction was associated with a sudden-permanent 0.44%
reduction in total crashes occurring during the hours of midnight to 5:00 a.m. for 15-
17-year-olds starting 1-year subsequent to the implementation of the nighttime
restriction. The results also suggested a marginally significant sudden-permanent
0.45% reduction in their nighttime fatal/injury crashes starting 1-year subsequent to
the program implementation. These effects translate into savings of 153 total crashes
and 68 fatal/injury crashes annually for 15-17-year-olds. These crash savings
estimates are based on an assumption that the GDL night driving restriction did not
increase daytime crashes.
• The 6-month passenger restriction was associated with a marginally significant
sudden-permanent 2.52% reduction in 15-17-year-old total teen passenger crashes,
and a significant gradual-permanent reduction stabilizing at -6.43% in fatal/injury
passenger crashes when using an intervention date 1-year subsequent to the
program start date. These effects equate to savings of 878 total crashes and 975
fatal/injury crashes annually for 15-17-year-olds. These crash savings estimates are
based on an assumption that the GDL passenger restriction did not cause an
increase in non-passenger crashes for the 15-17-year-old age group.
Discussion
The fact that no overall reductions were found in teen total or fatal/injury crash rates
from the program start date or from a 6-months subsequent date is not surprising given
the Williams, Nelson, and Leaf (2002) findings indicating that many teens were simply
applying for their instruction permit earlier to avoid delaying licensure, and that only
small increases were found in the percentages of teens receiving additional hours and
miles of supervised on-the-road practice during this longer instruction permit period. In
addition, the reductions associated with the nighttime and passenger restrictions were
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
v
small and occurred some months later in time and therefore would not have helped
detect an effect using the time periods analyzed for the overall analyses.
The fact that an increase was found in total crashes for 18-19-year-olds suggests that
GDL programs may have unintended negative consequences for this and possibly other
age groups. One possibility for this finding is that any positive effects of the program
may not continue into later years and that 16-17-year-olds under the program might not
be as safe and skilled at age 18 as they would have been without the GDL restrictions.
The increase in 18-19-year-old crash rates could also be due to a higher percentage of
that age group being licensed due to younger teens waiting to license until age 18 to
avoid the program. In any case, it is recommended that 18-19-year-olds not be used as a
comparison group for evaluations of GDL programs because it appears that drivers in
this age group are impacted by such programs.
Because the post-program crash rates for teens were compared to their pre-program
rates, and these pre-program rates already reflected the influence of crash reductions
associated with the original teen licensing program evaluated by Hagge and Marsh
(1988), any benefit of the program enhancements made in 1998 was expected to be only
marginal incremental reductions in crash rates. Indeed, the observed effects for the
nighttime and passenger restrictions were modest in size. If this evaluation had
compared the crash rates under this enhanced program with all of its components to
some theoretical set of teen crash rates for drivers under no program, it is much more
likely that significant and larger decreases in overall total and fatal/injury crash rates
associated with the program would have been found.
Finding reductions in total and fatal/injury nighttime crashes is consistent with results
from other states that have adopted nighttime restrictions (Ferguson, Leaf, Williams, &
Preusser, 1996; McKnight, Hyle, & Albrecht, 1983). The use of a 1-year delayed
intervention date for analyzing the effects of the nighttime and passenger restrictions
seems justified because it both reduced transition bias associated with the increase in
teen licensure around the time the enhancements were implemented and allowed time
for more teens to be fully under the program requirements. This latter issue is especially
relevant for evaluating the restrictions because they do not take effect until the teens
complete the 6-month instruction permit period. The percentage reductions associated
with the nighttime restriction were, however, quite small. Larger nighttime crash
reductions may have been realized if the nighttime restriction began at an earlier time
(e.g., 11:00 p.m. or earlier), as has been suggested by traffic safety experts (McKnight,
1986; Williams & Mayhew, 2003). In addition, although around 90% of teens complied
for at least the first 6 months of the restriction, only 60% of teens were found to have not
driven after midnight for their first full year after licensure (Williams et al., 2002).
Clearly if parents could be motivated to not permit driving for the full term of the
restriction, even larger reductions in nighttime teen crashes might be realized.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
vi
California was the first state to implement a meaningful teen passenger restriction
(Williams et al., 2002). Finding that the passenger restriction was associated with
modest, but significant reductions in both total and fatal/injury crashes is noteworthy
because it indicates that passenger restrictions are effective components of GDL
programs. Although compliance with the 6-month passenger restriction was not found
to be very high (around 50%), not transporting other teenagers during the first 6-
months of driving represented the largest actual change in behavior before and after the
GDL enhancements were implemented (Williams et al., 2002). Therefore it is not
surprising that the effects of the passenger restriction were larger than those for the
nighttime restriction. Given the high crash risk of teen drivers when they transport
other teenagers, finding ways to increase the willingness and ability of parents to
enforce the passenger restriction would likely result in additional crash savings.
Although the California GDL program evaluated in this report is considered to be one
of the strongest in the United States, there are additional features that could be added or
changed that may serve to strengthen the program even further. In addition to starting
the nighttime restriction at an earlier time and finding ways to increase compliance with
the nighttime and passenger restrictions, the program could be improved by making a
teen’s advancement from one stage of licensure to another contingent upon maintaining
a crash- and violation-free driving record, and by tying the passenger and nighttime
restrictions to the intermediate licensing stage rather than to a set period of time
(McKnight, 1986). Furthermore, compliance with the nighttime and passenger
restrictions could be increased by allowing law enforcement officers to stop teens
simply because they believe they are violating these restrictions (i.e., primary
enforcement).
Other authors (e.g., Mayhew & Simpson, 2002) have recommended that driver
education and training be integrated into GDL programs so that they are multi-staged,
with a basic driver education course before teens learn how to drive and an advanced
course after they have gained some experience driving on the road. More complex
topics, such as hazard perception, might be better taught in the advanced course where
experience on the road might make these topics more understandable. Results of a
recent evaluation (Masten & Chapman, 2003) showing that home-study driver
education courses were just as effective as classroom-based courses for teaching basic
driver education content may provide a means for removing some of the potential
roadblocks for integrating such a two-staged driver education and training system with
California’s GDL program. The use of home-study driver education for the first stage of
a tiered driver education and training program may also increase parental involvement
in their teen’s early driving experience, and motivate them to more fully enforce the
GDL restrictions.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
vii
TABLE OF CONTENTS
PAGE
PREFACE.......................................................................................................................... i
ACKNOWLEDGMENTS ................................................................................................ i
EXECUTIVE SUMMARY................................................................................................ i
Introduction................................................................................................................ i
Method........................................................................................................................ ii
Results......................................................................................................................... iii
Discussion................................................................................................................... iv
INTRODUCTION ............................................................................................................ 1
Licensing Programs for Teenagers ........................................................................... 1
California’s Licensing Program for Teenagers ........................................................ 2
METHODS........................................................................................................................ 6
Overview .................................................................................................................... 6
Data ............................................................................................................................. 7
Analyses...................................................................................................................... 8
RESULTS .......................................................................................................................... 12
Overall Total Crash Analyses ................................................................................... 12
Overall Fatal/Injury Crash Analyses ....................................................................... 15
Nighttime Total Crash Analyses............................................................................... 18
Nighttime Fatal/Injury Crash Analyses .................................................................. 21
Total Teen Passenger Crash Analyses...................................................................... 24
Fatal/Injury Teen Passenger Crash Analyses ......................................................... 27
Total Crashes for 16-Year-Olds................................................................................. 30
Fatal/Injury Crashes for 16-Year-Olds..................................................................... 32
Total and Fatal/Injury Crash Involvements............................................................ 33
Total Crashes for 18-19-Year-Olds............................................................................ 37
Fatal/Injury Crashes for 18-19-Year-Olds ............................................................... 40
DISCUSSION.................................................................................................................... 42
REFERENCES .................................................................................................................. 45
APPENDIX ....................................................................................................................... 48
LIST OF TABLES
NUMBER PAGE
1 Total Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions 14
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
viii
TABLE OF CONTENTS (continued)
LIST OF TABLES (continued)
NUMBER PAGE
2 Fatal/Injury Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions 17
3 Total Nighttime Crash Proportion Model Statistics for Gradual-
Permanent, Sudden-Temporary, and Sudden-Permanent Interventions 20
4 Fatal/Injury Nighttime Crash Proportion Model Statistics for
Gradual-Permanent, Sudden-Temporary, and Sudden-Permanent
Interventions 23
5 Total Teen Passenger Crash Proportion Model Statistics for
Gradual-Permanent, Sudden-Temporary, and Sudden-Permanent
Interventions 26
6 Fatal/Injury Teen Passenger Crash Proportion Model Statistics for
Gradual-Permanent, Sudden-Temporary, and Sudden-Permanent
Interventions 28
7 16-Year-Old Total Crash Rate Model Statistics for Gradual-
Permanent, Sudden-Temporary, and Sudden-Permanent
Interventions 31
8 16-Year-Old Fatal/Injury Crash Rate Model Statistics for Gradual-
Permanent, Sudden-Temporary, and Sudden-Permanent
Interventions 33
9 Total Involvement Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions 35
10 Fatal/Injury Involvement Crash Rate Model Statistics for
Gradual-Permanent, Sudden-Temporary, and Sudden-Permanent
Interventions 36
11 18-19-Year-Old Total Crash Rate Model Statistics for Gradual-
Permanent, Sudden-Temporary, and Sudden-Permanent
Interventions 39
12 18-19-Year-Old Fatal/Injury Crash Rate Model Statistics for Gradual-
Permanent, Sudden-Temporary, and Sudden-Permanent
Interventions 41
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
ix
TABLE OF CONTENTS (continued)
LIST OF FIGURES
NUMBER PAGE
1 Monthly counts of new provisional licenses issued and
total provisional licenses outstanding from January 1994
through December 2001 11
2 Monthly total crashes per 1,000 population for 15-17-year-olds
and 24-55-year-olds during January 1994 through December
2001 by age of youngest driver involved 13
3 Monthly fatal/injury crashes per 1,000 population for 15-17-year-olds
and 24-55-year-olds during January 1994 through December
2001 by age of youngest driver involved 16
4 Monthly total nighttime crashes per 1,000 15-17- year-olds
during January 1994 through December 2001 by age of
youngest driver involved 18
5 Proportion of monthly total crashes occurring during 12:00-5:00 a.m.
for 15-17-year-olds during January 1994 through December 2001
by age of youngest driver involved 19
6 Monthly fatal/injury nighttime crashes per 1,000 15-17-year-olds
during January 1994 through December 2001 by age of youngest
driver involved 21
7 Proportion of monthly fatal/injury crashes occurring during
12:00-5:00 a.m. for 15-17-year-olds during January 1994 through
December 2001 by age of youngest driver involved 22
8 Monthly total crashes with a passenger under age 20 per 1,000
15-17-year-olds during January 1994 through December 2001 by
age of youngest driver involved 24
9Proportion of monthly 15-17-year-old total crashes with a passenger
under age 20 during January 1994 through December 2001 by
age of youngest driver involved 25
10 Monthly fatal/injury crashes with a passenger under age 20
per 1,000 15-17-year-olds during January 1994 through December
2001 by age of youngest driver involved 27
11 Proportion of monthly 15-17-year-old fatal/injury crashes with a
passenger under age 20 during January 1994 through December
2001 by age of youngest driver involved 28
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
x
TABLE OF CONTENTS (continued)
LIST OF FIGURES (continued)
NUMBER PAGE
12 Monthly total crashes per 1,000 population for 16-year-olds and
24-55-year-olds during January 1994 through December 2001
by age of youngest driver involved 30
13 Monthly fatal/injury crashes per 1,000 population for 16-year-olds
and 24-55-year-olds during January 1994 through December 2001
by age of youngest driver involved 32
14 Monthly total crash involvements per 1,000 population for
15-17-year-olds and 24-55-year-olds during January 1994
through December 2001 34
15 Monthly fatal/injury crash involvements per 1,000 population for
15-17-year-olds and 24-55-year-olds during January 1994
through December 2001 34
16 Monthly total crashes per 1,000 population for 18-19-year-olds and
24-55-year-olds during January 1994 through December 2001
by age of youngest driver involved 38
17 Monthly fatal/injury crashes per 1,000 population for 18-19-year-olds
and 24-55-year-olds during January 1994 through December 2001
by age of youngest driver involved 40
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
1
INTRODUCTION
Teenage drivers have a much higher crash risk than do older drivers due to their
fundamental lack of driving skill, inexperience at driving, tendency towards increased
risk-taking, immaturity, inaccurate risk perception, and overestimation of driving skills
(Janke, Masten, McKenzie, Gebers, & Kelsey, 2003). States have tried to mitigate the
increased crash risk of teenagers by implementing modified driver licensing programs
for teenagers that focus on improving their skills and reducing their exposure to those
situations in which they are at the highest risk, such as driving at night or with young
passengers. The modified licensing systems for teenagers usually include several stages
leading to an unrestricted license. The licensing stages for teenagers typically include a
supervised practice period, license restrictions, and accelerated post-licensing control
actions that do not apply to adults. This report presents results of an evaluation of the
safety impact of several enhancements made in July 1998 to improve the effectiveness of
California’s licensing program for drivers under age 18.
Licensing Programs for Teenagers
‘Provisional Driver Licensing’ (PDL) and ‘Graduated Driver Licensing’ (GDL) are
common names for licensing programs designed for teenage drivers. Which term is
used typically depends on when the program was implemented (earlier programs
usually called PDL) or whether the program includes an intermediate licensing stage in
which teens are gradually exposed to riskier driving situations by sequentially
removing licensing restrictions (typically referred to as GDL). Because these two names
are often used interchangeably and all such programs are designed for teenage drivers,
the remainder of this report will usually not differentiate between the two.
Given that research on teen drivers has shown that increased driving experience is
associated with reduced crash risk (Ferguson, 1996; Mayhew & Simpson, 1990; Simpson
& Mayhew, 1992), many states, provinces, and countries have introduced licensing
programs for teenage drivers that gradually lift initial licensing restrictions to ease them
into higher risk driving situations (Foss & Goodwin, 2003; Mayhew & Simpson, 1984,
1996; McKnight, 1996; Shope & Molnar, 2003; Simpson, 2003). These programs may
include: (a) mandatory periods of supervised driving instruction on an instruction
permit, (b) restrictions from driving during certain hours at night, (c) restrictions from
carrying passengers under a certain age (usually age 20), (d) accelerated and more
severe penalties for drivers who violate traffic laws or cause crashes, and (e) zero-
tolerance or lower blood alcohol concentration (BAC) restrictions (Mayhew & Simpson,
1996; McKnight, 1996; Williams & Mayhew, 2003). The license restrictions and
accelerated post-licensing control actions are normally in effect during all or part of the
‘learner’ (instruction permit) and ‘intermediate’ (restricted) licensing stages. Some teen
licensing programs include minimum required hours of supervised driving practice, or
may lower the age or time requirements for obtaining an instruction permit or
intermediate license if the person has completed driver education and driver training
instruction.
The more stringent programs typically make advancement from one stage of licensing
to another contingent upon maintaining a crash- and violation-free driving record,
while other programs make advancement to the next stage based solely on time (e.g., a
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
2
12-month night driving restriction). Other authors have suggested additional
restrictions such as restricting novice teens from driving on freeways and during
weekends (McKnight, 1996; Mayhew & Simpson, 1984, 1996). These types of restrictions
are not as common, although they are supported by research findings (e.g., Cooper,
Pinili, & Chen, 1995). Restrictions on driving at night and transporting young
passengers are considered to be very important features of any teenage licensing
program, given the high crash risk for teenagers under these situations (Lin & Fearn,
2003; Williams & Mayhew, 2003). Night driving curfews have been shown to reduce
driving during the restricted hours and discourage early licensure (Williams, Lund, &
Preusser, 1985). Driving restrictions and curfews have been found to result in less risky
driving especially when licensure is contingent upon not receiving traffic violations
during the restricted stage (McKnight, 1986).
To date, 37 states have adopted comprehensive modified licensing programs for teens,
and 47 states and the District of Columbia have implemented one or more of the major
components mentioned above (Shope & Molnar, 2003). Programs in some jurisdictions
apply to new drivers of any age (e.g., Nova Scotia and Ontario), while others apply only
to novice drivers under certain ages (e.g., under age 25 in New Zealand and under age
18 in most U.S. states, including California). States that have adopted even some of the
key components, such as a nighttime restriction, have realized lower teenage crash rates
(Ferguson, Leaf, Williams, & Preusser, 1996; McKnight, Hyle, & Albrecht, 1983;
Preusser, Ferguson, & Williams, 1999). In fact, evaluations of these programs or their
components have generally found that they are associated with reductions in crashes,
although there is a lot of variation in the observed effect sizes (ranging from 4% to 60%).
The high variability is probably due to the fact that the programs differ in their
components, some being more comprehensive and strict than others, and to differences
in methodology used in the evaluations (e.g., different crash metrics and statistical
analyses). A fairly thorough summary of the results of a number of evaluations of
licensing programs for teenagers in various jurisdictions can be found in McKnight and
Peck (2002) and Masten (in press).
California’s Licensing Program for Teenagers
To obtain a learner’s permit in California, teens younger than age 18 must have
completed or be simultaneously enrolled in both driver education and driver training
courses or have completed driver education and be enrolled in a driver training course.
They also must pass vision and written knowledge tests. The minimum age to apply for
an instruction permit is 15 years. To obtain their driver license they must be at least 16
years of age and pass a drive test.
California’s first teen licensing program, implemented in October 1983, included the
following components for license applicants under age 18:
• A mandatory 1-month instruction permit period allowing driving only when
supervised by a licensed parent/guardian, spouse, or adult 25 years of age or older,
or a certified driving instructor.
• A parent/teen driver-practice guide that contains structured driving exercises that
the teen must master before taking a drive test.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
3
• A distinctive looking driver license, allowing easy identification of the driver as a
provisional licensee.
• A 1-week wait after failing the written knowledge test and 2-week wait after failing
the behind-the-wheel drive test before retesting.
• Parent certification that the teen successfully completed the exercises in the
parent/teen guide and is skilled enough to pass the DMV drive test.
• An accelerated post-licensing control action program in which teens receive a
warning letter after their first traffic violation or responsible crash, a 1-month
restriction allowing only supervised driving after their second violation or at-fault
crash in a 12-month period, a 6-month license suspension and 1-year probation after
a third offense in 12 months, and extended license suspension or possible revocation
after a fourth offense, violation of probation, failure to appear in court, or failure to
pay a fine.
Hagge and Marsh (1988) evaluated the California provisional licensing program using
time series analysis of statewide crash rates and also an assessment of individual driver
records. They found that the program as a whole was associated with 5.3% lower per
capita crash rates for 15-to-17-year-olds and 23% lower violation rates for 16-year-old
licensed drivers. Compared to the adult program, the accelerated post-licensing control
action program for teenagers was found to be superior for reducing subsequent 2-year
total crash and violation rates for teens, and increasingly more effective at higher point
counts. The findings also suggested that the accelerated program was more effective
than the adult program at reducing teen fatal/injury crash rates. Hagge and Marsh
judged that a reduction in driving exposure related to delaying licensure and the
1 month instruction permit requirement, and the earlier sanctioning of drivers violating
traffic laws and causing crashes, largely contributed to the safety benefits of California’s
provisional licensing program found in the study.
Even though the California program was found to reduce teen crash rates, teenage
drivers remained the single highest risk age group of California drivers after the
program was implemented (Aizenberg & McKenzie, 1997; Romanowicz & Gebers,
1990). In July 1998 the California Legislature enhanced the licensing program for
teenagers in response to the recalcitrant high crash risk of teenage drivers in California
and the fact that California’s licensing program for teenagers did not include some of
the components more recently considered by experts to be essential for an optimal
program, such as nighttime and passenger restrictions. In addition, the California
Legislature implemented a zero-tolerance alcohol law in 1994, which results in a 1-year
license suspension for anyone under the age of 21 apprehended while driving with a
BAC of 0.01% or higher, and has had primary enforcement of its mandatory seatbelt
law since 1993. The enhanced California teen licensing program (now called GDL) is
considered to be quite comprehensive and contains all of the components of an optimal
system, with the exception of making advancement from one stage of licensing to
another contingent upon maintaining a crash- and violation-free driving record
(Williams & Mayhew, 2003).
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
4
In addition to having to pass the vision, written, and drive tests, the California
graduated licensing program evaluated in this report (California Vehicle Code Section
12814.6; see the Appendix) includes all of the components of the original provisional
licensing program identified above plus:
• A minimum 6-month instruction permit period.
• Parent/guardian certification that the teen driver completed a minimum of 50 hours
of behind-the-wheel practice (10 hours of which must be at night) supervised by a
licensed parent/guardian, spouse, adult 25 years of age or older, or certified driving
instructor.
• A 12-month restriction from driving between 12:00 a.m. and 5:00 a.m., unless
supervised as defined above. Exceptions are granted for medical or family necessity,
school activities, and employment needs, with a note signed by the proper authority
such as a parent or principal and specifying the ending date for the exception.
• A 6-month restriction from driving with passengers under the age of 20, unless
supervised as defined above. Exceptions are allowed under the same circumstances
indicated above.
A survey of California teens and their parents regarding the new teen licensing system
suggested widespread support for the program enhancements by parents whose teens
were subject to the new requirements and restrictions (Williams, Nelson, & Leaf, 2002).
These authors surveyed two groups of teenagers who were applying for a driver license
in California, as well as their parents. One group was sampled from April through June
1998, right before the July 1998 program implementation date, and the other was
sampled from May through July 1999. The first group was not subject to the new
program requirements because only persons who applied for an instruction permit on
or after July 1, 1998 were subject to the law changes. The second group of teenagers was
subject to the new program requirements. Compliance with the enhanced program
provisions is essential for them to be effective. The results of their survey indicated
widespread, although not universal, compliance with holding the instruction permit for
6 months (97%), and the 50 hours of supervised instruction requirement (81%—10 hours
at night 79%). However, much lower compliance was reported by the parents
regarding the 1-year nighttime driving restriction (59%), and the 6-month passenger
restriction (52%). Not surprisingly, the parents tended to report more compliance with
the requirements and restrictions than did their teenagers.
As stated above, compliance with the new requirements is important for the enhanced
program to reduce crash rates. However, the findings in the Williams et al. (2002)
survey indicate that a significant percentage of parents were already imposing similar
requirements and restrictions on their teenagers before July 1998, which has major
implications for the results of this evaluation. In terms of percentage-point increase,
their findings show that only 20% more teens held their instruction permit for 6 months
or longer after the enhancements (97%) compared to those who applied before the
program (77%). In addition, only 14% more drove the 50-hours of required supervised
practice (67% before vs. 81% after), and only 10% more at night (69% before vs. 79%
after). These increases, although statistically significant, indicate that a high percentage
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
5
of parents were already requiring these things of their teenagers before the
enhancements were implemented. Similarly, the percentage of parents indicating that
they did not allow their teens to drive after midnight for the full 1 year (54%) was only
5 percentage points higher after July 1998 (59%). Only for the passenger restriction was
the percentage-point gain (38%) much higher after the law change (14% before vs. 52%
after), but overall compliance was not very high for either of the new restrictions.
It should be noted that enforcement of the nighttime and passenger restrictions is not
likely to be high because law enforcement officers are not permitted to stop teens solely
for violating these restrictions (secondary enforcement), and because the penalties for
violations are not very severe. In fact, such violations do not count as negligent operator
points on the driver record and do not result in post-licensing control actions. Instead,
violations of the restrictions are handled administratively by judges, who may impose 8
to 16 hours of community service or a $35 fine for a first offense and 16 to 24 hours of
community service or a $50 fine for subsequent offenses. These factors conceivably
could tend to decrease the commitment of teenagers to obey, and parents to enforce, the
restrictions.
One additional finding from their survey with implications for the current evaluation is
that the program does not appear to have resulted in a delay of licensure. Specifically,
teenage drivers applying for a license before and after the program enhancements did
not differ in the average age at licensure (the average age being 16 years and 6 months
for both groups). This may seem surprising given that the instruction permit period was
extended from 1 to 6 months, but it can probably be largely explained by the fact that
teens applied for their instruction permits 3 months earlier on average after the
program than before. This allowed them to obtain their provisional license at the same
age on average as those who applied before the instruction permit period was extended.
Together, these findings suggest that teens drove earlier and had a higher risk exposure
level after the program started. Recall that the reduction in driving exposure due to
delayed licensure was judged to be one of the most important factors for the
effectiveness of California’s initial teen licensing program evaluated by Hagge and
Marsh (1988); this factor does not appear to be in play in the current evaluation.
The longer period of driving on an instruction permit would not be expected to result in
significantly higher crash rates because supervised driving is generally considered to be
of low risk (Williams, 2003; Williams, Preusser, Ferguson, & Ulmer, 1997). In fact, the
longer instruction permit period could arguably decrease crash risk because it would
have allowed teens to gain more supervised driving practice and become more skillful
before they obtained their license. Regarding gaining more supervised driving
experience, as mentioned above the Williams et al. (2002) survey results showed that
only a slightly higher percentage of parents (14 percentage points higher) reported that
their children practiced for at least 50 hours after the program enhancements were
implemented (81%) compared to beforehand (67%). In addition, there was only a 22
percentage-point increase in the number of teens reporting that they practiced driving
for 500 miles or more after the program enhancements were implemented (52%)
compared to beforehand (30%). These results do not suggest that the longer instruction
permit period resulted in widespread increased supervised driving practice as might
have been hoped. Given the likely nominal positive effect this minimal increased
practice would be expected to have, the fact that teens were driving (albeit supervised)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
6
at an earlier age on average, and the finding that the program did not result in a delay
of licensure (i.e., an older average licensing age), it is unlikely that the longer instruction
permit period had much of a positive safety benefit.
The low overall levels of compliance with the passenger and nighttime driving
restrictions and the modest increases in the percentages of parents restricting their teen
drivers after the enhancements, along with a similarly modest increase in the
percentage of parents requiring additional supervised practice, would not lead one to
believe that the program enhancements would have had a substantial impact on the
crash rates of teen drivers. In addition, because the crash rates of teens analyzed in this
evaluation already reflect the influence of reductions associated with the original teen
licensing program evaluated by Hagge and Marsh, any benefit of the program
enhancements made in 1998 would be expected to be marginal.
Although this report presents the official California Department of Motor Vehicles’
evaluation of the enhanced California GDL program, other evaluations of the program
have been completed. Results of two evaluations of California’s program by the
Automobile Club of Southern California suggested positive results of the program
(Bloch, 2000; Bloch, Shin, & Labin, 2002). However, the analysis methods used in these
evaluations did not adequately adjust for preexisting trend in the crash data and used
questionable methodology. Results of another evaluation of the California program
sponsored by the California State Automobile Association (Atkins, Cooper, & Gillen,
2002) also suggested reductions in crashes caused by 16-year old drivers associated with
the program, but the methods used in that study were of questionable validity and
must therefore also be considered equivocal. Finally, a quasi-experimental pre-post
comparison of 16-year-old drivers in San Diego County, California by Smith, Pierce,
Ray, and Murrin (2001) did not find changes in the per-driver crash rates for 16-year-
olds following the implementation of the GDL law, although their results did suggest a
20% decrease in per-capita crash and injury rates. However, the authors did not account
for preexisting trend in the data that might explain the observed per-capita reduction.
The current evaluation is considered superior to the four just mentioned primarily
because it uses an analytical approach that accounts for trend and seasonality in the
data before any potential effect of the program enhancements is evaluated.
METHODS
Overview
Monthly statewide per capita crash rates for January 1994 to December 2001 were
analyzed using Autoregressive Integrated Moving Average (ARIMA) intervention time
series analysis to determine whether implementing the GDL enhancements in July 1998
changed the rate of crashes involving 15-to-17-year-old drivers in California. The crash
rates for adult drivers aged 24 to 55 were used as a control series in some of the analyses
to account for history-related factors that would have affected crashes for both age
groups. The collection of time series data and analysis procedures are described below.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
7
Data
Teenage drivers aged 15 to 17 years comprised the treatment group for purposes of the
evaluation, and adult drivers aged 24 to 55 years were used as a control group. All
drivers aged 15 to 17 who applied after the program start date were subject to the GDL
program requirements during the period in which they hold an instruction permit or
provisional driver license. Age 24 was chosen as the lower bound for the control group
because 24-year-olds were the youngest drivers who would not have been subject to the
new GDL program during the time period analyzed in the evaluation. The criterion
measures used to evaluate the overall impact of the GDL program are different types of
crashes per population unit for 15-to-17-year-old drivers, and in some cases separately
for 16-year-old and 18-19-year-old drivers. To evaluate the effect of the restrictions,
additional analyses were conducted to determine whether the percentage
representation of crashes occurring during 12:00-5:00 a.m., or involving passengers
under age 20, changed after the GDL enhancements.
The traffic crash data used in this evaluation were obtained from the California
Highway Patrol’s (CHP’s) Statewide Integrated Traffic Records System (SWITRS).
Given that the GDL program was implemented the first day of July 1998, the first 54
months (January 1994 to June 1998) represented the pre-implementation time period for
the analyses, while the last 42 months (July 1998 to December 2001) comprised the post-
implementation time period. To create the per capita crash rates, counts of crashes
involving at least one driver of a passenger vehicle, pickup, or motorcycle were
assigned to an age group based on the age of the youngest driver involved in the crash.
If multiple driver age groups were involved, the crash was attributed to the youngest
driver.
Annual population data by age were obtained from the California Department of
Finance for the years 1993 to 2002. Monthly population counts were interpolated from
the annual counts by assuming linear increases and decreases in the population across
all 12 months of a given year. Per capita crash rates were computed by dividing the
monthly number of crashes for each age group by the monthly estimated population in
the age group.
The following criterion crash series were created and analyzed in this evaluation:
1. Total crashes
2. Fatal/injury crashes
3. Proportion of total crashes occurring during 12:00-5:00 a.m.
4. Proportion of fatal/injury crashes occurring during 12:00-5:00 a.m.
5. Proportion of total crashes involving passengers under age 20
6. Proportion of fatal/injury crashes involving passengers under age 20
7. Total crashes involving 16-year-olds
8. Fatal/injury crashes involving 16-year-olds
9. Total crashes involving 18-19-year-olds
10. Fatal/injury crashes involving 18-19-year-olds
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
8
The first two series were analyzed to evaluate the impact of GDL as a whole. The
analyses of crashes in which a 16-year-old was the youngest involved driver are
conceptually less biased for purposes of evaluating the impact of the GDL
enhancements, because of the shorter transition time period for all drivers in this age
group to be completely under the new GDL program requirements. The four series
involving proportions of crashes during the restricted time period and involving
passengers less than 20 years of age were used to evaluate the impact of the nighttime
restriction and passenger restriction components of GDL, respectively. The analyses of
18-19-year-old drivers in crashes were conducted to determine if the program had any
positive or negative effects on this age group.
Two additional crash series not listed above were analyzed. These consisted of crash
involvements for which a single crash incident was typically assigned multiple times
(one “crash” count assigned to each driver involved). These series were analyzed to
determine if using crash involvement rates would yield results consistent with those
from the analysis of total crash rates based on the age of the youngest involved driver.
One analysis was conducted for total crash involvements and another was conducted
for fatal/injury crash involvements.
Although it would have been desirable to also analyze crash rates per driver (including
those who were not fully licensed) and rates per licensed driver, it was not possible to
calculate these rates because the crash volumes from SWITRS include crashes for
permit-holding, licensed, and unlicensed drivers while DMV has counts of licensed
drivers only.
Analyses
ARIMA intervention time series analysis (McCleary & Hay, 1982) was used to create
mathematical models that best described the crash rates of the 15-to-17-year-old drivers
using auto-regressive (AR), integrated (I), and moving average (MA) components. The
full multivariate model developed for some of the criterion-measure series included the
corresponding crash rate series of 24-to-55 year olds as a covariate to reduce bias in the
teen series caused by temporary or long-term effects of historical events other than the
GDL program. It was assumed for purposes of the analysis that such extraneous factors
(e.g., changes in general traffic safety laws, traffic and weather conditions, etc.) would
influence both age groups equally. A good ARIMA model would account for any
seasonal fluctuations and upward or downward trend that might otherwise obscure, or
be mistaken for, a program effect. After the appropriate ARIMA model parameters
were identified and estimated and parameters representing any other explanatory (or
control) variables were added to the model, the intervention effect was evaluated by
adding additional model parameters representing the intervention. Because ARIMA
intervention analysis accounts for trend and seasonality in the data, it provides more
statistical power and a less biased assessment of the intervention effect than do other
techniques based on linear regression models.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
9
The multivariate time series models included the following four additive components
after any necessary differencing of the time series data was performed:
1. the covariate (adult) series multiplied by a coefficient designated β;
2. a multiplicative combination of autoregressive (AR) and/or moving average (MA)
factors that best described the seasonal and nonseasonal variation—trends, cycles,
autocorrelations, and so forth—in the treatment (15-17-year-old) series that was not
accounted for by the covariate (this variance being commonly referred to as ‘noise’);
3. an intervention component that characterized the hypothetical effect of the program
on the treatment series; and
4. error, that portion of variance in the dependent variable that remained unexplained
(which would be minimized by a best-fitting model).
The final ARIMA models created in this study included (with the exception of the
transition elements discussed later in this section) only parameters that were
statistically significant at an alpha level of .05 (i.e., those that had less than a 5%
likelihood of being found to be significant due to chance alone). Meta-diagnosis of
competing alternative ARIMA models was completed to ensure that the most
parsimonious models were chosen.
The teen and adult total and fatal/injury crash series were first modeled as separate
univariate series to determine whether there were any changes in the series’ levels
coinciding with the start of GDL. Then, to evaluate the overall impact of GDL, the teen
total and fatal/injury crash series were modeled using the adult series as a covariate (or
explanatory variable) to account for common variance in the two series (such as might
be due to reduced driving in both age groups resulting from increases in gasoline prices
for example). Underlying the use of the adult series as a control for the teen series is the
assumption that changes in the behavior of the adult series represents a baseline level of
what would have been expected in the teen series in the absence of the GDL program.
Because the adult series would be expected to model additional variability in the teen
series beyond the univariate ARIMA structure, together with its functioning as a control
series, the multivariate models represent a more powerful and valid assessment of GDL
program impact than do the univariate models.
After the ARIMA models for either the univariate or multivariate series were
completed, three different sets of parameters were added to each model one at a time to
test three different hypotheses of the intervention’s impact on teen crash rates:
(a) gradual-permanent, (b) sudden-temporary, and (c) sudden-permanent. All three
types of interventions were evaluated for goodness-of-fit to see which, if any, best
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
10
modeled the impact of GDL. The direction, size, and statistical significance of the
intervention parameter estimates defined the nature of the shift, if any, in the level of
the treatment series caused by the GDL program.
The impact of the GDL program was expected to occur gradually and be permanent to
reflect the gradual increase in the proportion of individuals who fell under the
requirements of the GDL program over time. This gradual infusion of GDL drivers
would result from the exclusion from the GDL program requirements (including the
nighttime and passenger restrictions) of applicants who applied for a license before July
1998, even though they received their license after that date. By July 2001, all licensed
16-to-17-year-old drivers were subject to the requirements of GDL. Although a gradual-
permanent change in the teen series was the most logical expected GDL effect, sudden-
temporary and sudden-permanent impact models were also assessed.
In addition to the gradual increase in the proportion of teen drivers under the program
over time, implementing GDL resulted in two other factors that likely affected the teen
crash rates: (a) some teens may have applied earlier than they normally would have to
avoid being in the program, and (b) some teens may have delayed licensure until age 18
to avoid being in the program. Figure 1 shows counts of monthly new provisional
licenses issued and counts of outstanding provisional licenses from January 1994 to
December 2001. Monthly counts of provisional licenses outstanding were interpolated
from biannual counts by assuming linear increases in licenses issued and decreases in
drivers turning age 18 across each 6-month period.
As can be seen in the figure, much higher volumes of provisional licenses were issued
immediately before GDL was implemented, at which time the total number of
outstanding provisional licenses also increased dramatically, remained higher than
usual, and then dropped lower than was the case before the program was implemented.
Simply put, the transition resulted in having more teen drivers on the road for some
time immediately before and after the date that the GDL law was implemented. This
sudden increase in licensed driver volumes likely affected teen crash rates and may
have caused confounding transition effects in the analyses. Therefore, it would be
expected that teen crash rates would temporarily increase for the periods immediately
before and after GDL was implemented, all else being equal. In an attempt to account
for this transition effect, separate parameter estimates were added to the time series
models to account for possible changes in the series’ levels during the 6 months before,
6 months after, and in some cases the second 6-month period after July 1998. The
effective intervention points evaluated in these transition models were therefore
January 1, 1999 (6 months after the actual implementation date) and in some cases July
1, 1999 (12 months after the intervention date). Incidentally, the survey findings of
Williams et al. (2002) showing no change in the average age at licensure would likely
not have been affected by this temporary increase in license volumes because their
survey time frame occurred well after the rush of new licensees.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
11
MONTHLY NEW PROVISIONAL LICENSES ISSUED
MONTHLY PROVISIONAL LICENSES OUTSTANDING
Figure 1. Monthly counts of new provisional licenses issued and total provisional licenses
outstanding from January 1994 through December 2001.
250,000
260,000
270,000
280,000
290,000
300,000
310,000
320,000
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
GDL Enhancements
(July 1998)
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
12
The parameter structure representing the gradual-permanent intervention effect was
ω
δ
1−
, where ω represents the treatment effect in units of crash rate the first month
following intervention, and δ quantifies how quickly a stable impact was realized
during subsequent months (the larger the value, the longer it took to reach an
asymptote). The total change in the series level due to intervention for this effect was
estimated as
ω
δ
1−
. The parameter structure representing the sudden-temporary effect
was the same, except that
ω
δ
1−
represented the total displacement of the series level
(e.g., the total volume of crashes saved during the period before the crash rate series
returned to its preexisting level). The transfer function for the sudden-permanent
intervention component was indicated by ω, which reflected the average change in
series level after intervention. The existence of a change in the level of the series
subsequent to intervention (i.e., a program impact) is indicated by the presence of a
statistically significant ω parameter in the intervention component. Negative ω
parameters indicate a decrease in crashes subsequent to GDL, whereas positive values
indicate that the crash rates increased. When there is statistically significant evidence of
program impact, the statistical significance of δ is assessed to determine how well the
theoretical change model (sudden or gradual) fits the data. If ω is not significant, the
effect is considered to be nonsignificant. In addition, if δ is negative it indicates an
unstable oscillating effect that could not be reasonably argued to have been caused by
the program, and in this event the intervention effect modeled would be rejected.
(According to McDowall, McCleary, Meidinger, and Hay [1980], a value of δ that is
negative indicates that the time series system is unstable.)
RESULTS
Overall Total Crash Analyses
Plots of monthly total crashes per 1,000 population for 15-17-year-olds and 24-55-year
olds are shown in Figure 2. The implementation of the GDL enhancements on July 1,
1998 is represented by the vertical dashed line shown in the figure.
Visual inspection of the data suggests that the total crash rates for 15-17-year-olds
steadily declined throughout the time series period, with the exception of a temporary
increase immediately before and after the program was implemented. The adult series
appears to have remained flat throughout the same time period. There does not appear
to be a significant change in either series coinciding with July 1, 1998, although the teen
rates appear higher for the periods 6-months before and after the program. It should be
emphasized that it can be very difficult to judge whether or not there was a change in
the series based solely on a visual inspection of the data. What may appear to the naked
eye as an intervention effect may prove to be nonsignificant when analyzed by the
appropriate statistical test. On the other hand, where there does not appear to be an
intervention effect, a statistically significant change may have indeed occurred. It is
even more difficult to visually detect differences between two or more time series.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
13
Figure 2. Monthly total crashes per 1,000 population for 15-17-year-olds and 24-55-year-
olds during January 1994 through December 2001 by age of youngest driver involved.
The teen and adult series also evidence a seasonal pattern of data corresponding to
yearly cycles, meaning that points 12 months apart are correlated and the series
changed level within annual cycles. The lowest crash rates tended to occur in January
and February, and the highest rates generally occurred between September and
December. The average teen crash rate was 1.93 (SD = 0.17), and extreme values were
1.64 for February 2001 and 2.47 for October 1994. The average rate for adults was 1.41
(SD = 0.07), about 27% lower than the teen average, and extreme values were a low of
1.22 for February 1997 and a high of 2.49 for December 1995.
Table 1 presents the model statistics for the teen and adult univariate series and the
multivariate teen series wherein the adult crash series was used to control for variability
in the teen series. To reduce any bias associated with the transition effect discussed in
the Methods section, additional parameters were entered into the multivariate model to
represent behavior in the crash rates during the 6 months preceding and 6 months
following the program start date. In these models, the intervention was made to start 6
months after the July 1998 formal start date of the program (i.e., January 1999).
For simplicity, model diagnostic statistics (e.g., Ljung-Box Q and residual mean square
error) are not shown. All final models chosen met the common criteria for acceptability
and were judged to be the most parsimonious and to give the best fit to the data of all
models considered. The transition parameters were kept in the models regardless of
their level of statistical significance, while all other parameters were evaluated using an
alpha level of .05, two-tailed.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
TOTAL CRASHES PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
Ages 24-55
Ages 15-17
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
14
Table 1
Total Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter Lag Estimate t
Teen univariate
Gradual-permanentaIntervention ω0 0.0919 1.53
δ1 -0.8825 -7.26*
Noise MA 1 0.4482 4.56*
MA 12 0.7074 8.22*
Sudden-temporaryaIntervention ω0 0.0553 1.46
δ1 -0.8390 -4.52*
Noise MA 1 0.4621 4.68*
MA 12 0.6127 6.53*
Sudden-permanentaIntervention ω0 0.0223 0.27
Noise MA 1 0.4588 4.63*
MA 12 0.6997 8.45*
Adult univariate
Gradual-permanentbIntervention ω0 -0.0360 -1.83
δ1 -1.0000 -37.89*
Noise MA 12 0.6903 7.87*
Sudden-temporarybIntervention ω0 0.0212 0.41
δ1 -0.7116 -0.34
Noise MA 12 0.7073 8.25*
Sudden-permanentbIntervention ω0 -0.0308 -2.59*
Noise MA 12 0.7340 9.07*
Teen multivariate
July 1998 intervention point
Gradual-permanentaIntervention ω0 -0.0046 -0.07
δ1 -0.3013 -0.02
Covariate β0 1.0081 8.42*
Noise MA 1 0.4221 3.97*
MA 12 0.6041 6.27*
Sudden-temporaryaIntervention ω0 -0.0126 -0.20
δ1 0.0182 0.00
Covariate β0 1.0044 7.80*
Noise MA 1 0.4173 3.91*
MA 12 0.6026 6.30*
Sudden-permanentaIntervention ω0 0.0013 0.02
Covariate β0 1.0073 8.42*
Noise MA 1 0.4224 3.99*
MA 12 0.6060 6.34*
January 1999 intervention point
Gradual-permanentaIntervention ω0 0.1720 1.65
δ1 -0.5303 -1.78
Covariate β0 0.9900 8.49*
Noise MA 1 0.4233 3.89*
MA 12 0.6106 6.17*
January 98-June 98 Transition 0 0.1058 1.92
July 98-December 98 Transition 0 0.0762 1.35
Sudden-temporaryaIntervention ω0 0.2305 3.08*
δ1 0.7630 5.68*
Covariate β0 1.0219 8.03*
Noise MA 1 0.6006 5.97*
MA 12 0.6008 6.15*
January 98-June 98 Transition 0 0.1261 2.70*
July 98-December 98 Transition 0 0.1583 2.67*
Sudden-permanentaIntervention ω0 0.1557 1.45
Covariate β0 1.0029 8.44*
Noise MA 1 0.4446 4.22*
MA 12 0.6115 6.30*
January 98-June 98 Transition 0 0.1117 1.92
July 98-December 98 Transition 0 0.1170 1.39
aModels included differencing at lags 1 and 12 to produce stationary residuals. bDifferenced at lag 12.
*p < .05, two-tailed.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
15
The only statistically significant intervention component in the models that did not
include transition effect components was the sudden-permanent effect in the univariate
model for the 24-55-year-old total crash series (ω = -0.0308, t = -2.59), indicating an
average monthly reduction of 0.0308 crashes per 1,000 24-55-year-olds. This represents
about a 2.17% decrease in the average adult crash rate after implementation of the GDL
enhancements, which equals 484 fewer crashes per month, or about 5,811 crashes saved
per year.
When the 6-month transition components were included in the multivariate model, a
significant sudden-temporary effect was found, with ω = 0.2305 (t = 3.08) and δ = 0.7630
(t = 5.68). This represents an increase of 0.2305 crashes per 1,000 15-17-year-olds the first
month after the January 1999 intervention date (an 11.51% increase), with smaller
monthly increases occurring until the series level returned to its pre-intervention level.
The total one-time increased crash volume displacement is equal to
ω
δ
1−
or 0.9726
crashes per 1,000 capita. Given that the average monthly population of 15-17-year-olds
from January 1999 to December 2001 was 1,455,160, this translates into a one-time cost
of about 1,415 additional total crashes.
In summary, neither the univariate nor multivariate teen analyses found a statistically
significant permanent change in the 15-17-year-old crash rates after the enhancements
were introduced. However, a sudden-temporary increase in crashes occurred in the
multivariate 15-17-year-old series after transition components for the 6 months before
and 6 months after the program start date were added to the model, moving the
intervention point forward to January 1999. This measured increase could be due to a
rise in crash rates due to the influx of teen drivers that continued for some time beyond
the transition periods included in the model.
Overall Fatal/Injury Crash Analyses
Monthly fatal/injury crashes per 1,000 population for 15-17-year-olds and 24-55-year-
olds are plotted in Figure 3. The vertical line in the figure again indicates the
implementation of the GDL enhancements in July 1998.
Both series are very similar in pattern to the total crash rate series, as would be expected
given that fatal/injury crashes are a subset of total crashes. The rates for both groups,
but particularly those for teens, appear to have generally steadily declined during the
period of January 1994 to December 2001. The teen series again appears to have
temporarily increased in the 6-month periods before and after the July 1998 program
implementation date. In general, the crash rate for each group was lowest during the
first few months of each year, gradually increased until September or October, and then
dropped. The robust overall downward trend in the teen series strongly supports the
analytical strategy used here to evaluate intervention effects only after removing
seasonality and trend in the data.
The average teen fatal/injury crash rate was 0.82 (SD = 0.10), about 1.37 times the
overall average rate of 0.60 (SD = 0.04) for adults. Extreme values for teens were 0.65 for
February 2001 and 1.13 for October 1994. The highest rate for adults was 0.69 for
October 1994, and the lowest rate was 0.51 for February 1999.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
16
Figure 3. Monthly fatal/injury crashes per 1,000 population for 15-17-year-olds and 24-55-
year-olds during January 1994 through December 2001 by age of youngest driver involved.
Table 2 presents the model statistics for each type of intervention effect for the teen and
adult univariate series and the multivariate teen series wherein the adult crash rates
were used to control for extraneous variability in the teen crash series. The multivariate
model was again also evaluated using additional parameters to account for transitional
behavior in the series 6-months before and after the formal start date. A January 1999
intervention date (6-months after the actual program implementation date) was used
for this multivariate analysis. The statistical significance of all parameters was
determined using an alpha level of .05, but the transition parameters were kept in the
models regardless of their level of statistical significance.
The estimate of ω was not statistically significant for any of the series or types of
intervention effects. Therefore, the null hypothesis of no intervention effect on
fatal/injury crashes was accepted for all the analyses. Results of the multivariate
analyses including transition components representing 6 months before and 6 months
after the July 1998 start date to remove any bias associated with the transition effect also
did not indicate a significant change in the teen fatal/injury crash series. Specifically, no
sudden-temporary increase in fatal/injury crashes at the January 1999 intervention date
was found, which was not consistent with the effect found for total crashes. In
summary, the results do not suggest that the July 1998 enhancements were associated
with any temporary or permanent changes in the fatal/injury crash rates of 15-17-year-
olds, even after controlling for extraneous variability shared with the adult series,
adding parameters to remove bias associated with the transition effect, and moving the
intervention point 6-months ahead to January 1999.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
FATAL/INJURY CRASHES
PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
Ages 24-55
Ages 15-17
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
17
Table 2
Fatal/Injury Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter La
g
Estimate t
Teen univariate
Gradual-permanent Intervention ω0 -0.0105 -0.27
δ1 0.5705 0.29
Noise MA 1 0.4988 5.08*
MA 12 0.5693 6.13*
Sudden-temporary Intervention ω0 -0.0188 -0.52
δ1 0.3708 0.18
Noise MA 1 0.5004 5.17*
MA 12 0.5636 5.93*
Sudden-permanent Intervention ω0 0.0007 0.02
Noise MA 1 0.5103 5.19*
MA 12 0.5763 5.64*
Adult univariate
Gradual-permanent Intervention ω0 0.0106 0.50
δ1 -0.7726 -0.64
Noise MA 1 0.6261 7.20*
MA 12 0.5919 6.18*
Sudden-temporary Intervention ω0 0.0114 0.80
δ1 -0.6529 -1.65
Noise MA 1 0.6725 7.92*
MA 12 0.6168 5.93*
Sudden-permanent Intervention ω0 0.0023 0.13
Noise MA 1 0.6234 7.13*
MA 12 0.5930 6.32*
Teen multivariate
July 1998 intervention point
Gradual-permanent Intervention ω0 -0.0047 -0.12
δ1 -0.4956 -0.05
Covariate β0 0.4569 2.40*
Noise MA 1 0.5340 5.34*
MA 12 0.5632 5.96*
Sudden-temporary Intervention ω0 -0.0293 -0.84
δ1 0.3476 0.26
Covariate β0 0.4697 2.29*
Noise MA 1 0.4778 4.85*
MA 12 0.5247 5.35*
Sudden-permanent Intervention ω0 -0.0059 -0.17
Covariate β0 0.4421 2.34*
Noise MA 1 0.5124 5.03*
MA 12 0.5223 5.35*
January 1999 intervention point
Gradual-permanent Intervention ω0 0.0543 0.95
δ1 -0.1436 -0.20
Covariate β0 0.4024 2.23*
Noise MA 1 0.6182 6.47*
MA 12 0.4981 4.85*
January 98-June 98 Transition 0 0.0809 2.78*
July 98-December 98 Transition 0 0.0900 2.24*
Sudden-temporary Intervention ω0 0.0591 1.43
δ1 0.8318 2.91*
Covariate β0 0.4005 2.04*
Noise MA 1 0.6199 6.54*
MA 12 0.4979 4.83*
January 98-June 98 Transition 0 0.0791 3.02*
July 98-December 98 Transition 0 0.0849 2.71*
Sudden-permanent Intervention ω0 0.0503 0.95
Covariate δ0 0.4032 2.24*
Noise MA 1 0.6185 6.48*
MA 12 0.4921 4.88*
January 98-June 98 Transition 0 0.0814 2.79*
July 98-December 98 Transition 0 0.0897 2.23*
Note. All models included differencing at lags 1 and 12 to produce stationary residuals.
*p < .05, two-tailed.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
18
Nighttime Total Crash Analyses
Monthly nighttime (12:00-5:00 a.m.) total crashes per 1,000 15-17-year-olds are plotted in
Figure 4. The teen crash series suggests that nighttime crashes were the highest during
the Summer months (June-August) when teens were out of school. The rates appear to
have declined up until around the time the GDL enhancements were enacted and then
leveled out for about a 1-year period. One-year after the intervention point, the teen
rates appear to have declined again. The average teen nighttime total crash rate was
0.09 (SD = 0.02). Extreme values were 0.06 for January 2000 and 0.15 for August 1994.
Figure 4. Monthly total nighttime crashes per 1,000 15-17-year-olds during January 1994
through December 2001 by age of youngest driver involved.
To estimate the impact of the GDL enhancements (particularly the nighttime driving
restriction) on nighttime crash rates, the crashes represented in Figure 4 were evaluated
as monthly proportions of 15-17-year-old total crashes. These monthly nighttime crash
proportions are shown in Figure 5. The proportion of total teen crashes occurring
during nighttime hours appears to have declined very slowly throughout the time
series period, and the highest proportions again occur during the Summer months
(June-August). The proportion representation of nighttime crashes for teen drivers
ranged from 0.0032 of total crashes for October 2000 to 0.0700 for August 1994, with a
mean representation of 0.0464 (SD = 0.0080).
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
TOTAL NIGHTTIME CRASHES
PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
19
Figure 5. Proportion of monthly total crashes occurring during 12:00-5:00 a.m. for 15-17-
year-olds during January 1994 through December 2001 by age of youngest driver involved.
Table 3 presents the univariate model statistics for each type of intervention effect
evaluated for the teen nighttime proportion total crash series. The statistical significance
of all parameters was again determined using an alpha level of .05. The three types of
interventions were evaluated for three different sets of analyses presented in the table.
The first set of analyses was calculated using the actual July 1998 implementation date
as the intervention point for the models. To account for the transition effect, the second
set of analyses included two transition components (6 months before and 6 months after
the actual implementation date) to account for the increase in 16-17-year-old licensees
around the time the GDL enhancements were implemented. For these analyses, the
intervention date used was January 1999, which is 6 months after the GDL
enhancements were actually implemented. The third set of analyses included these two
6-month transition components, plus an additional 6-month transition component,
extending the intervention date by 1 year (to July 1999). This was done because no 16-
17-year-olds would be subject to the nighttime restriction until at least 6 months after
the implementation date due to the mandatory 6-month instruction permit period, and
the additional 6 months allows for additional time for a reasonable number of teen
drivers to be subject to the restriction. The transition parameters were kept in the
models regardless of their level of statistical significance, while all other parameters
were evaluated using an alpha level of .05, two-tailed.
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
PROPORTION OF TOTAL
CRASHES OCCURRING AT NIGHT
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
20
Table 3
Total Nighttime Crash Proportion Model Statistics for Gradual-Permanent, Sudden-
Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter Lag Estimate t
July 1998 Intervention Point
Gradual-permanent Intervention ω0 -0.0050 -1.72
δ1 -0.7419 -1.28
Noise MA 12 0.5068 4.70*
Sudden-temporary Intervention ω0 -0.0094 -1.64
δ1 -0.4990 -0.98
Noise MA 12 0.4402 4.29*
Sudden-permanent Intervention ω0 -0.0024 -1.37
Noise MA 12 0.4985 4.73*
January 1999 intervention point
Gradual-permanent Intervention ω0 -0.0008 -0.50
δ1 0.8160 2.02*
Noise MA 12 0.5018 4.95*
January 98-June 98 Transition 0 -0.0026 -1.03
July 98-December 98 Transition 0 -0.0020 -0.76
Sudden-temporary Intervention ω0 0.0025 0.41
δ1 0.2176 0.12
Noise MA 12 0.4288 4.00*
January 98-June 98 Transition 0 -0.0010 -0.40
July 98-December 98 Transition 0 0.003 0.13
Sudden-permanent Intervention ω0 -0.0036 -1.81
Noise MA 12 0.4863 4.67*
January 98-June 98 Transition 0 -0.0028 -1.11
July 98-December 98 Transition 0 -0.0016 -0.63
July 1999 intervention point
Gradual-permanent Intervention ω0 -0.0073 -2.18*
δ1 -0.9883 -2.07*
Noise MA 12 0.5085 5.06*
January 98-June 98 Transition 0 -0.0023 -0.89
July 98-December 98 Transition 0 -0.0019 -0.73
January 99-June 99 Transition 0 -0.0015 -0.54
Sudden-temporary Intervention ω0 0.0014 0.23
δ1 -0.3172 -0.07
Noise MA 12 0.4844 4.87*
January 98-June 98 Transition 0 -0.0009 -0.34
July 98-December 98 Transition 0 0.0000 0.00
January 99-June 99 Transition 0 0.0008 0.30
Sudden-permanent Intervention ω0 -0.0044 -2.12*
Noise MA 12 0.5003 4.91*
January 98-June 98 Transition 0 -0.0025 -0.95
July 98-December 98 Transition 0 -0.0020 -0.79
January 99-June 99 Transition 0 -0.0017 -0.62
Note. All models included differencing at lag 12 to produce stationary residuals.
*p < .05, two-tailed.
The estimate of ω was not statistically significant for any of the intervention types for
the July 1998 or January 1999 nighttime total crash proportion models. However, the
estimate of ω was statistically significant for the gradual-permanent (ω = -0.0073) and
sudden-permanent (ω = -0.0044) models using the July 1999 implementation date. The
gradual-permanent effect hypothesis was rejected because the δ parameter was
negative, indicating an unstable effect. The sudden-permanent effect (ω = -0.0044)
equals a 9.27% drop from the 0.0475 average proportional series level before the July
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
21
1999 intervention date. This represents a total monthly savings of 0.0087 crashes per
1,000 15-17-year-olds. Based on the average monthly population of 15-17-year-olds from
July 1999 to December 2001 of 1,461,604, this amounts to about 13 crashes saved per
month or 153 crashes saved annually, which is only a 0.44% reduction in total per capita
crashes. This crash savings estimate is based on an assumption that the GDL night
driving restriction did not increase daytime crashes.
Nighttime Fatal/Injury Crash Analyses
Monthly fatal/injury nighttime (12:00-5:00 a.m.) crashes per 1,000 15-17-year-olds are
plotted in Figure 6. The fatal/injury crash series again suggests that the highest
nighttime crash rate for teens was during the Summer months (June-August). However,
October also tended to have a high nighttime fatal/injury teen crash rate, although this
was not the case for nighttime total crashes. The rates appear to have declined until
around April 2000 and then leveled. The average teen nighttime fatal/injury crash rate
was 0.04 (SD = 0.01). Extreme low values for teens were 0.02 for February 1997, 1998,
and 2000, and October 2000. The highest fatal/injury nighttime rate for teens was 0.08
for August and October 1994.
Figure 6. Monthly fatal/injury nighttime crashes per 1,000 15-17-year-olds during January
1994 through December 2001 by age of youngest driver involved.
The impact of the nighttime restriction was estimated based on an analysis of the
proportion of fatal/injury crashes that occurred during the night curfew hours. The
proportions of 15-17-year-old fatal/injury crashes occurring at nighttime (12:00-5:00
a.m.) each month are shown in Figure 7. Note the two extreme outlying proportions for
August 1994 and July 2001.
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
NIGHTTIME FATAL/INJURY CRASHES
PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
22
Figure 7. Proportion of monthly fatal/injury crashes occurring during 12:00-5:00 a.m. for 15-
17-year-olds during January 1994 through December 2001 by age of youngest driver
involved.
The proportion of fatal/injury teen crashes occurring at night appears to decline very
slowly during January 1994 to around September 2000, at which point it appears to
increase. The highest proportions again occur during June through August. Nighttime
fatal/injury crashes ranged from 0.0030 of all fatal/injury crashes for October 2000 to
0.0850 for August 1994, with a mean proportion of 0.0460 (SD = 0.0100) of all
fatal/injury crashes.
Table 4 presents the model statistics for each type of intervention effect for the teen
fatal/injury nighttime proportion series. The statistical significance of all parameters
was again determined using an alpha level of .05. Three different sets of analyses are
again presented in the table. The first are based on the July 1998 intervention date, the
second are based on a January 1999 intervention date, and the third are based on a July
1999 intervention date. The latter two sets of analyses again include parameters to
reduce bias associated with the transition effect, and the final includes an additional 6-
month transition component. In addition, two pulse parameters were used to model the
extreme outliers for August 1994 and July 2001 in all models shown in Table 4. The
transition and outlier pulse parameters were kept in the models regardless of their level
of statistical significance, while all other parameters were evaluated using an alpha
level of .05, two-tailed.
The estimate of ω was not statistically significant for any of the intervention types and
intervention points for nighttime fatal/injury crash proportions. However, the estimate
of ω = -0.0045 was borderline statistically significant (t = -1.94) for the sudden-
permanent intervention for the July 1999 (1-year-post implementation) series. This
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
PROPORTION OF FATAL/INJURY
CRASHES OCCURRING AT NIGHT
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
23
result suggests that there was a sudden 9.56% drop from the average 0.0471 prior series
level in the proportion of nighttime fatal/injury crashes beginning 1-year after the GDL
program implementation date. This equals a monthly savings of 0.0039 fatal/injury
crashes per 1,000 15-17-year-olds, which is about 6 per month or 68 fatal/injury crashes
annually. The decrease was not very large, however, translating into only a 0.45%
decrease in all per capita fatal/injury crashes for 15-17-year-olds. Again, this crash
savings estimate is based on an assumption that the GDL night driving restriction did
not increase daytime crashes.
Table 4
Fatal/Injury Nighttime Crash Proportion Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter La
g
Estimate t
July 1998 intervention point
Gradual-permanent Intervention ω0 -0.0045 -1.34
δ1 -0.8068 -1.17
Noise MA 12 0.5666 6.04*
August 1994 outlier 0 0.0271 4.10*
July 2001 outlier 0 0.0209 2.61*
Sudden-temporary Intervention ω0 -0.0045 -0.92
δ1 -0.5553 -1.08
Noise MA 12 0.5592 5.95*
August 1994 outlier 0 0.0276 4.28*
July 2001 outlier 0 0.0204 2.61*
Sudden-permanent Intervention ω0 -0.0024 -1.20
Noise MA 12 0.5711 6.06*
August 1994 outlier 0 0.0269 4.05*
July 2001 outlier 0 0.0212 2.64*
January 1999 intervention point
Gradual-permanent Intervention ω0 -0.0010 -0.45
δ1 0.7776 1.22
Noise MA 12 0.5585 5.92*
January 98-June 98 Transition 0 -0.0034 -1.12
July 98-December 98 Transition 0 -0.0003 -0.09
August 1994 outlier 0 0.0268 4.06*
July 2001 outlier 0 0.0223 2.80*
Sudden-temporary Intervention ω0 -0.0077 -1.12
δ1 -0.5208 -0.73
Noise MA 12 0.5514 5.81*
January 98-June 98 Transition 0 -0.0021 -0.75
July 98-December 98 Transition 0 0.0018 0.66
August 1994 outlier 0 0.0272 4.26*
July 2001 outlier 0 0.0212 2.75*
Sudden-permanent Intervention ω0 -0.0040 -1.83
Noise MA 12 0.5533 5.95*
January 98-June 98 Transition 0 -0.0037 -1.20
July 98-December 98 Transition 0 -0.0002 -0.05
August 1994 outlier 0 0.0269 4.08*
July 2001 outlier 0 0.0220 2.78*
July 1999 intervention point
Gradual-permanent Intervention ω0 -0.0032 -0.73
δ1 0.3269 0.38
Noise MA 12 0.5544 5.91*
January 98-June 98 Transition 0 -0.0035 -1.14
July 98-December 98 Transition 0 -0.0004 -0.13
January 99-June 99 Transition 0 -0.0026 -0.81
August 1994 outlier 0 0.0268 4.09*
July 2001 outlier 0 0.0226 2.80*
Sudden-temporary Intervention ω0 0.0016 0.23
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
24
Table 4 (continued)
Intervention Model component Parameter Lag Estimate t
δ1 -0.5438 -0.17
Noise MA 12 0.5539 5.82*
January 98-June 98 Transition 0 -0.0021 -0.71
July 98-December 98 Transition 0 0.0017 0.61
January 99-June 99 Transition 0 -0.0003 -0.10
August 1994 outlier 0 0.0273 4.20*
July 2001 outlier 0 0.0215 2.68*
Sudden-permanent Intervention ω0 -0.0045 -1.94
Noise MA 12 0.5563 5.96*
January 98-June 98 Transition 0 -0.0034 -1.12
July 98-December 98 Transition 0 -0.0004 -0.13
January 99-June 99 Transition 0 -0.0025 -0.79
August 1994 outlier 0 0.0269 4.10*
July 2001 outlier 0 0.0222 2.80*
Note. All models included differencing at lag 12 to produce stationary residuals.
*p < .05, two-tailed.
Total Teen Passenger Crash Analyses
The monthly total crashes per 1,000 15-17-year-olds involving a passenger under the
age of 20 are shown in Figure 8 for the period between January 1994 through December
2001.
Figure 8. Monthly total crashes with a passenger under age 20 per 1,000 15-17-year-olds
during January 1994 through December 2001 by age of youngest driver involved.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
TOTAL PASSENGER CRASHES
PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
25
The teen passenger total crash rates appear to be relatively flat before the GDL
enhancements, increase immediately afterward, decrease starting about a year later,
and then increase again during the last year. The mean teen total passenger crash rate
for the entire period shown in the figure was 0.6922 per 1,000 capita (SD = 0.0786).
The impact of the passenger restriction was estimated based on an analysis of the
proportion of total teen crashes that involved a passenger under the age of 20. The
proportion of teen passenger crashes out of all teen crashes is plotted in Figure 9 for the
period January 1994 through December 2001. The proportion representation of teen
passenger crashes also appears to have declined about a year or so after the GDL
enhancements were implemented. The mean total crash proportion for teens during this
period was 0.3590 (SD = 0.0212).
Figure 9. Proportion of monthly 15-17-year-old total crashes with a passenger under age 20
during January 1994 through December 2001 by age of youngest driver involved.
The gradual-permanent, sudden-temporary, and sudden-permanent intervention types
for these data were modeled using three different intervention points: July 1998,
January 1999, and July 1999. The first intervention point represented the actual date the
passenger restriction was implemented (although no teens driving at that time would
have been subject to the restriction due to the 6-month instruction permit period). The
second 6-month subsequent time point was used to enable 6-month pre- and 6-month
post-transition components to be embedded in the model to remove the transition
effects of having an large increase in the number of teens licensed around the time of
the GDL enhancements. The final 1-year subsequent time point was used to determine
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
PROPORTION OF TOTAL CRASHES
WITH A PASSENGER UNDER AGE 20
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
26
the effects of the passenger restriction after at least half a year’s worth of teens were
subject to the passenger restriction. The time series models for each of the intervention
time points and intervention types are shown in Table 5. The transition parameters
were kept in the models regardless of their level of statistical significance, while all
other parameters were evaluated using an alpha level of .05, two-tailed.
Table 5
Total Teen Passenger Crash Proportion Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter Lag Estimate t
July 1998 intervention point
Gradual-permanent Intervention ω0 0.0043 0.33
δ1 -0.3365 -0.16
Noise MA 1 0.0805 8.50*
Sudden-temporary Intervention ω0 0.0039 0.34
δ1 0.9655 0.57
Noise MA 1 0.6599 8.34*
Sudden-permanent Intervention ω0 0.0024 0.25
Noise MA 1 0.6778 9.19*
January 1999 intervention point
Gradual-permanent Intervention ω0 -0.0008 -0.05
δ1 0.5909 0.04
Noise MA 1 0.6901 9.20*
January 98-June 98 Transition 0 0.0017 0.18
July 98-December 98 Transition 0 0.0063 0.50
Sudden-temporary Intervention ω0 0.0147 1.06
δ1 0.9054 2.05*
Noise MA 1 0.6876 8.89*
January 98-June 98 Transition 0 0.0049 0.55
July 98-December 98 Transition 0 0.0127 1.17
Sudden-permanent Intervention ω0 -0.0049 -0.33
Noise MA 1 0.6943 9.42*
January 98-June 98 Transition 0 0.0003 0.03
July 98-December 98 Transition 0 0.0035 0.28
July 1999 intervention point
Gradual-permanent Intervention ω0 0.0001 0.01
δ1 -0.8531 -0.01
Noise MA 1 0.7340 10.68*
January 98-June 98 Transition 0 0.0020 0.23
July 98-December 98 Transition 0 0.0065 0.56
January 99-June 99 Transition 0 0.0012 0.09
Sudden-temporary Intervention ω0 0.0192 1.51
δ1 -0.0558 -0.09
Noise MA 1 0.7012 9.49*
January 98-June 98 Transition 0 0.0077 0.95
July 98-December 98 Transition 0 0.0175 1.89
January 99-June 99 Transition 0 0.0178 2.03*
Sudden-permanent Intervention ω0 -0.0252 -1.91
Noise MA 1 0.7797 12.16*
January 98-June 98 Transition 0 0.0010 0.12
July 98-December 98 Transition 0 0.0044 0.43
January 99-June 99 Transition 0 -0.0022 -0.18
Note. All models including differencing at lag 1 to produce stationary residuals.
*p < .05, two-tailed.
As can be seen in the table, none of the ω parameters were statistically significant for
any of the time periods, indicating no significant gradual or sudden decrease in the
proportion of teen passenger crashes at the intervention point, 6-months afterward, or
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
27
1-year subsequent. However, the t value for the sudden-permanent model 1-year
subsequent to the GDL enhancements (at which time about a half year’s worth of teens
would be subject to the passenger restriction) was borderline significant (ω = -0.0252,
t = -1.91). This suggests the possibility that the proportion of passenger crashes dropped
by 0.0252 or 6.82% from the January 1994 through June 1999 average proportion of
0.3692. This equates to an average monthly savings of 0.0501 passenger crashes per
1,000 15-17-year-olds, which is approximately 73 crashes per month or 878 crashes per
year. This effect represents a 2.52% decrease in per capita total crashes (whether or not
they involved a passenger under age 20). This crash savings estimate is based on an
assumption that the GDL passenger restriction did not cause an increase in non-
passenger crashes for the 15-17-year-old age group.
Fatal/Injury Teen Passenger Crash Analyses
The monthly fatal/injury crashes per 1,000 15-17-year-olds involving a passenger under
age 20 are shown in Figure 10 for the period between January 1994 through December
2001. The mean fatal/injury passenger crash rate for teens during this period was 0.3709
(SD = 0.0556). The impact of the passenger restriction was again estimated based on an
analysis of the proportion of fatal/injury teen crashes that involved a passenger under
the age of 20. These proportions are shown in Figure 11 for the same time period. The
mean fatal/injury teen passenger crash proportion during this period was 0.4491
(SD = 0.0278). The time series model estimates, again analyzed using the actual,
6-month post, and 1-year subsequent intervention points, are shown in Table 6. The
transition parameters were again kept in the models regardless of their level of
statistical significance, while all other parameters were evaluated using an alpha level
of .05, two-tailed.
Figure 10. Monthly fatal/injury crashes with a passenger under age 20 per 1,000 15-17-year-
olds during January 1994 through December 2001 by age of youngest driver involved.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
FATAL/INJURY PASSENGER
CRASHES PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
28
Figure 11. Proportion of monthly 15-17-year-old fatal/injury crashes with a passenger under
age 20 during January 1994 through December 2001 by age of youngest driver involved.
Table 6
Fatal/Injury Teen Passenger Crash Proportion Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter Lag Estimate t
July 1998 intervention point
Gradual-permanent Intervention ω0 -0.0026 -1.94
δ1 0.9791 38.11*
Noise MA 1 0.8910 16.67*
MA 3 -0.3943 -3.96*
Sudden-temporary Intervention ω0 -0.0003 -0.31
δ1 -1.0724 -14.45*
Noise MA 1 0.7702 11.36*
MA 3 -0.3941 -3.98*
Sudden-permanent Intervention ω0 -0.0073 -0.63
Noise MA 1 0.7628 11.27*
MA 3 -0.3821 -3.80*
January 1999 intervention point
Gradual-permanent Intervention ω0 -0.0048 -2.29*
δ1 0.9374 27.97*
Noise MA 1 0.9107 19.53*
MA 3 -0.3928 -3.99*
January 98-June 98 Transition 0 -0.0023 -0.26
July 98-December 98 Transition 0 -0.0028 -0.31
Sudden-temporary Intervention ω0 0.0039 0.23
δ1 0.8407 0.32
Noise MA 1 0.7730 10.78*
MA 3 -0.3548 -3.40*
January 98-June 98 Transition 0 0.0024 0.22
July 98-December 98 Transition 0 0.0063 0.47
0.00
0.10
0.20
0.30
0.40
0.50
0.60
PROPORTION OF FATAL/INJURY CRASHES
WITH A PASSENGER UNER AGE 20
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
29
Table 6 (continued)
Intervention Model component Parameter Lag Estimate t
January 1999 intervention point (continued)
Sudden-permanent Intervention ω0 -0.0228 -1.28
Noise MA 1 0.7771 11.45*
MA 3 -0.3942 -3.87*
January 98-June 98 Transition 0 -0.0066 -0.56
July 98-December 98 Transition 0 -0.0104 -0.69
July 1999 intervention point
Gradual-permanent Intervention ω0 -0.0100 -2.03*
δ1 0.8447 11.29*
Noise MA 1 0.9077 18.77*
MA 3 -0.4228 -4.31*
January 98-June 98 Transition 0 -0.0034 -0.37
July 98-December 98 Transition 0 -0.0052 -0.50
January 99-June 99 Transition 0 -0.0166 -1.49
Sudden-temporary Intervention ω0 0.0189 1.25
δ1 0.6925 0.76
Noise MA 1 0.7925 12.58*
MA 3 -0.3825 -3.78*
January 98-June 98 Transition 0 0.0032 0.31
July 98-December 98 Transition 0 0.0074 0.63
January 99-June 99 Transition 0 0.0025 0.21
Sudden-permanent Intervention ω0 -0.0244 -1.23
Noise MA 1 0.7864 11.18*
MA 3 -0.3815 -3.82*
January 98-June 98 Transition 0 -0.0064 -0.55
July 98-December 98 Transition 0 -0.0101 -0.67
January 99-June 99 Transition 0 -0.0214 -1.19
Note. All models included differencing at lag 1 to produce stationary residuals.
*p < .05, two-tailed.
The gradual-permanent intervention model was statistically significant 6-months
(ω = -0.0048, t = -2.29) and 1-year (ω = -0.0100, t = -2.03) post-implementation and
approached significance (ω = -0.0026, t = -1.94) at the actual July 1998 intervention point.
Given that no teenagers were actually under the passenger restriction for at least 6
months after the actual implementation date, and the fact that the July 1999 intervention
model represents at least partial saturation of teens under the restriction and hence a
more accurate representation of the true impact of the passenger restriction than the
January 1999 intervention model, the estimation of series reductions in percentage
representation of fatal/injury passenger crashes and fatal/injury crash savings are
based on the July 1999 intervention model. This effect indicates that the proportion of
fatal/injury passenger crashes decreased by 0.010 or 2.16% from the January 1994
through June 1999 average proportion of 0.4630 the first month after the July 1999
intervention date (a 1.0% decrease in all fatal/injury crashes). Larger monthly decreases
occurred until the series reached a stable post-intervention level of 0.0644 fewer
proportional passenger crashes per 1,000 capita (a 13.91% decrease in the proportional
representation of passenger fatal/injury crashes). This equates to an average monthly
savings of 0.0556 fatal/injury passenger crashes per 1,000 15-17-year-olds, or a 6.43%
decrease in all 15-17-year-old fatal/injury crashes based on the 0.8644 pre-intervention
level. Given that the average monthly population of 15-17-year olds from July 1999 to
December 2001 was 1,461,604, this translates into about 81 fewer fatal/injury crashes
per month, or 975 fewer fatal/injury crashes annually. Again, this crash savings
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
30
estimate is based on an assumption that the GDL passenger restriction did not cause an
increase in non-passenger crashes for the 15-17-year-old age group.
Total Crashes for 16-Year-Olds
Recall that analyses involving 16-year-olds as the youngest involved driver are
considered to be conceptually less biased for purposes of evaluating the overall impact
of the GDL enhancements, because of a shorter transition time period for the drivers in
this age group to be completely under the new GDL program. Plots of total monthly
crashes per 1,000 16-year-olds and 24-55-year-olds during the period of January 1994
through December 2001 are shown in Figure 12. The average total crash rate per 1,000
16-year-olds during this time period was 2.31 (SD = 0.30), which is 0.38 higher than the
15-17-year-old total crash rate analyzed earlier, and 1.64 times higher than the adult rate
of 1.41.
Figure 12. Monthly total crashes per 1,000 population for 16-year-olds and 24-55-year-olds
during January 1994 through December 2001 by age of youngest driver involved.
The results of the time series analyses for the gradual-permanent, sudden-temporary,
and sudden-permanent intervention types for the series for 16-year-olds, both with and
without using the adult rate as a covariate, are shown in Table 7. Although any bias
associated with the transition effects would be expected to be lower for 16-year-olds
than for the analyses including 15-17-year-olds conducted earlier, the analyses for 16-
year-olds were also conducted using two different possible intervention points, July
1998 and January 1999, in an attempt to remove any such effects. The transition
parameters were kept in the models regardless of their level of statistical significance,
while all other parameters were evaluated using an alpha level of .05, two-tailed. The
adult univariate rates are not modeled here because they were modeled earlier.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
TOTAL CRASHES
PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
Ages 24-55
Age 16
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
31
Table 7
16-Year-Old Total Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter La
g
Estimate t
Teen univariate
Gradual-permanent Intervention ω0 0.0494 0.41
δ1 0.6757 0.59
Noise MA 1 0.3978 3.89*
MA 12 0.6543 7.22*
Sudden-temporary Intervention ω0 -0.0497 -0.46
δ1 -0.2269 -0.11
Noise MA 1 0.4176 4.21*
MA 12 0.6083 6.60*
Sudden-permanent Intervention ω0 0.0480 0.43
Noise MA 1 0.4043 4.00*
MA 12 0.6579 7.45*
Teen multivariate
July 1998 intervention point
Gradual-permanent Intervention ω0 0.0687 0.74
δ1 0.3294 0.39
Covariate β0 0.9323 4.88*
Noise MA 1 0.4027 3.89*
MA 12 0.6171 6.69*
Sudden-temporary Intervention ω0 -0.0881 -0.94
δ1 -0.3766 -0.43
Covariate β0 0.9788 4.86*
Noise MA 1 0.3669 3.52*
MA 12 0.6102 6.64*
Sudden-permanent Intervention ω0 0.0316 0.32
Covariate β0 0.9308 4.84*
Noise MA 1 0.3917 3.85*
MA 12 0.6208 6.76*
January 1999 intervention point
Gradual-permanent Intervention ω0 0.0975 1.22
δ1 -0.9125 -6.29*
Covariate β0 0.9217 4.78*
Noise MA 1 0.4223 3.91*
MA 12 0.6187 6.71*
January 98-June 98 Transition 0 0.0651 0.85
July 98-December 98 Transition 0 0.1112 1.26
Sudden-temporary Intervention ω0 0.1311 0.89
δ1 0.6202 1.25
Covariate β0 0.9494 4.62*
Noise MA 1 0.4570 4.15*
January 98-June 98 Transition 0 0.0741 0.82
July 98-December 98 Transition 0 0.1298 1.07
Sudden-permanent Intervention ω0 0.0594 0.34
Covariate β0 0.9181 4.72*
Noise MA 1 0.4382 4.16*
MA 12 0.6254 6.68*
January 98-June 98 Transition 0 0.0654 0.68
July 98-December 98 Transition 0 0.1137 0.84
Note. All models included differencing at lags 1 and 12 to produce stationary residuals.
*p < .05, two-tailed.
Consistent with the results found for the total crash series including all teens aged 15-
17-years-old, the results for 16-year-olds did not indicate a significant permanent
change in their total crash rates after the GDL enhancements either at the actual
implementation date or 6-months subsequent. This was found even when the 24-55-
year-old series was used to remove variability in the 16-year-old total crash rates. The
only difference between these analyses and those for the 15-17-year-olds is that the 16-
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
32
year-old series did not experience a statistically significant temporary increase in total
crash rates 6-months after the GDL program was implemented. This finding reinforces
the conclusion that this temporary increase in total crashes for the 15-17-year-olds is
really just an artifact of the transition effect.
Fatal/Injury Crashes for 16-Year-Olds
Plots of fatal/injury monthly crashes per 1,000 16-year-olds and 24-55-year-olds during
January 1994 through December 2001 are shown in Figure 13. The average fatal/injury
crash rate per 1,000 16-year-olds during this time period was 0.98 (SD = 0.16), which is
0.16 times higher than the 15-17-year-olds fatal/injury crash rate analyzed earlier, and
1.63 times higher than the adult fatal/injury per-capita rate of 0.60.
Figure 13. Monthly fatal/injury crashes per 1,000 population for 16-year-olds and 24-55-year-
olds during January 1994 through December 2001 by age of youngest driver involved.
The time series analysis results for this series are shown in Table 8. The transition
parameters were kept in the models regardless of their level of statistical significance,
while all other parameters were evaluated using an alpha level of .05, two-tailed.
Consistent with the analyses conducted for 15-17-year-olds, none of the intervention
parameters were statistically significant for 16-year-old fatal/injury crashes at the actual
July 1998 implementation date or 6-months subsequent, even after using the adult series
as a covariate. This indicates that the program implementation was not associated with
a statistically significant sudden or gradual change in the fatal/injury crash rates of 16-
year-olds.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
FATAL/INJURY CRASHES
PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
Ages 24-55
Age 16
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
33
Table 8
16-Year-Old Fatal/Injury Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter La
g
Estimate t
Teen univariate
Gradual-permanentaIntervention ω0 0.0220 0.48
δ1 0.1951 0.20
Noise MA 1 0.4492 4.30*
MA 12 0.5620 5.80*
Sudden-temporaryaIntervention ω0 -0.0964 -1.81
δ1 -0.1449 -0.30
Noise MA 1 0.4070 4.00*
MA 12 0.5789 6.06*
Sudden-permanentaIntervention ω0 -0.0125 -0.21
Noise MA 1 0.4366 4.26*
MA 12 0.5644 5.89*
Teen multivariate
July 1998 intervention point
Gradual-permanentbIntervention ω0 0.0122 0.23
δ1 -0.3234 -0.17
Covariate β0 1.2273 4.63*
Noise MA 1 0.4490 4.35*
Sudden-temporarybIntervention ω0 -0.1093 -1.57
δ1 -0.0952 -0.15
Covariate β0 1.2616 4.79*
Noise MA 1 0.3856 3.78*
Sudden-permanentbIntervention ω0 -0.0256 -0.35
Covariate β0 1.2442 4.69*
Noise MA 1 0.4386 4.45*
January 1999 intervention point
Gradual-permanentbIntervention ω0 -0.1172 -0.98
δ1 -0.2868 -0.37
Covariate β0 1.2630 4.69*
Noise MA 1 0.5478 5.70*
January 98-June 98 Transition 0 0.0232 0.35
Jul
y
98-December 98 Transition 0 0.0328 0.36
Sudden-temporarybIntervention ω0 -0.0529 -0.64
δ1 0.1305 0.09
Covariate β0 1.3654 4.97*
Noise MA 1 0.5062 4.94*
January 98-June 98 Transition 0 0.0514 0.88
Jul
y
98-December 98 Transition 0 0.0695 1.03
Sudden-permanentbIntervention ω0 -0.1045 -0.92
Covariate β0 1.2464 4.67*
Noise MA 1 0.5399 5.55*
January 98-June 98 Transition 0 0.0222 0.33
Jul
y
98-December 98 Transition 0 0.0292 0.31
aModels included differencing at lags 1 and 12 to produce stationary residuals. bDifferenced at lag 1.
*p < .05, two-tailed.
Total and Fatal/Injury Crash Involvements
For the following analyses, series were created in which each crash incident was
typically assigned multiple times, because most crashes involved two or more drivers.
These analyses were conducted to determine if using crash involvement rates would
yield results consistent with those from the analyses of total and fatal/injury crash rates
based on the age of the youngest involved driver. Plots of total and fatal/injury
monthly crashes per 1,000 population for 15-17-year-olds and 24-55-year-olds are
shown in Figures 14 and 15, respectively.
The average total crash involvement rate was 1.93 (SD = 0.17) for 15-17-year-olds and
1.85 (SD = 0.09) for 24-55-year-olds. This teen total involvement rate is only 1.04 times
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
34
higher than the adult rate. The average fatal/injury crash involvement rate was 0.82
(SD = 0.10) for teens and 0.80 (SD = 0.05) for adults. The teen fatal/injury involvement
rate was again only slightly higher (1.03) than the adult fatal/injury involvement rate.
Figure 14. Monthly total crash involvements per 1,000 population for 15-17-year-olds and
24-55-year-olds during January 1994 through December 2001.
Figure 15. Monthly fatal/injury crash involvements per 1,000 population for 15-17-year-olds
and 24-55-year-olds during January 1994 through December 2001.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
TOTAL CRASH INVOLVEMENTS
PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
Ages 24-55
Ages 15-17
GDL Enhancements
(July 1998)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
FATAL/INJURY CRASH
INVOLVEMENT PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
Ages 24-55
Ages 15-17
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
35
The results of the time series for both the total and fatal/injury crash involvement series
included univariate estimates of gradual-permanent, sudden-temporary, and sudden-
permanent intervention effects for both the 15-17-year-old and 24-55-year-old series.
These three intervention effects were also evaluated in multivariate series in which the
24-55-year-old crash involvements were used to remove variability in the 15-17-year-old
crash involvement rates. The first multivariate series uses the actual July 1998 GDL
enhancement implementation date as an intervention point. The second multivariate
series uses a January 1999 (6-months subsequent) intervention point and includes
parameters for the 6-months before and 6-months after the implementation date to
remove any bias associated with the transition effect. The results for the total crash
involvement series are shown in Table 9 and the results for the fatal/injury crash
involvement series are shown in Table 10. The transition parameters were kept in the
models regardless of their level of statistical significance, while all other parameters
were evaluated using an alpha level of .05, two-tailed.
Table 9
Total Involvement Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter Lag Estimate t
Teen univariate
Gradual-permanentaIntervention ω0 0.0919 1.53
δ1 -0.8825 -7.26*
Noise MA 1 0.4482 4.56*
MA 12 0.7074 8.22*
Sudden-temporaryaIntervention ω0 0.0553 1.46
δ1 -0.8390 -4.52*
Noise MA 1 0.4621 4.68*
MA 12 0.6127 6.53*
Sudden-permanentaIntervention ω0 0.0223 0.27
Noise MA 1 0.4588 4.63*
MA 12 0.6997 8.45*
Adult univariate
Gradual-permanentbIntervention ω0 -0.0136 -0.21
δ1 0.1593 0.02
Noise MA 12 0.7153 8.55*
Sudden-temporarybIntervention ω0 0.0166 0.25
δ1 -0.8949 -0.29
Noise MA 12 0.7003 8.84*
Sudden-permanentbIntervention ω0 -0.0151 -0.83
Noise MA 12 0.7087 8.32*
Teen multivariate
July 1998 intervention point
Gradual-permanentaIntervention ω0 -0.0075 -0.12
δ1 -0.4315 -0.06
Covariate β0 0.8175 9.42*
Noise MA 1 0.4223 3.93*
MA 12 0.5630 5.73*
Sudden-temporaryaIntervention ω0 -0.0149 -0.25
δ1 0.0604 0.02
Covariate β0 0.8198 8.72*
Noise MA 1 0.4136 3.85*
MA 12 0.5706 5.93*
Sudden-permanentaIntervention ω0 0.0001 0.00
Covariate β0 0.8174 9.43*
Noise MA 1 0.4217 3.94*
MA 12 0.5702 5.83*
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
36
Table 9 (continued)
Intervention Model component Parameter Lag Estimate t
January 1999 intervention point
Gradual-permanentaIntervention ω0 0.1505 1.52
δ1 -0.5570 -1.77
Covariate β0 0.7989 9.39*
Noise MA 1 0.4281 3.90*
MA 12 0.5715 5.77*
January 98-June 98 Transition 0 0.0951 1.82
July 98-December 98 Transition 0 0.0937 1.29
Sudden-temporaryaIntervention ω0 0.2257 3.29*
δ1 0.7539 5.93*
Covariate β0 0.8315 8.93*
Noise MA 1 0.6274 6.28*
MA 12 0.5743 5.85*
January 98-June 98 Transition 0 0.1176 2.73*
July 98-December 98 Transition 0 0.1604 2.96*
Sudden-permanentaIntervention ω0 0.1290 1.26
Covariate β0 0.8075 9.36*
Noise MA 1 0.4439 4.16*
MA 12 0.5745 5.86*
January 98-June 98 Transition 0 0.0979 1.77
July 98-December 98 Transition 0 0.1023 1.27
aAll models included differencing at lags 1 and 12 to produce stationary residuals. bDifferenced at lag 12.
*p < .05, two-tailed.
Table 10
Fatal/Injury Involvement Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter Lag Estimate t
Teen univariate
Gradual-permanent Intervention ω0 -0.0105 -0.27
δ1 0.5705 0.29
Noise MA 1 0.4988 5.08*
MA 12 0.5693 6.13*
Sudden-temporary Intervention ω0 -0.0188 -0.52
δ1 0.3708 0.18
Noise MA 1 0.5004 5.17*
MA 12 0.5636 5.93*
Sudden-permanent Intervention ω0 0.0007 0.02
Noise MA 1 0.5103 5.19*
MA 12 0.5763 5.64*
Adult univariate
Gradual-permanent Intervention ω0 0.0147 0.55
δ1 -0.8195 -1.16
Noise MA 1 0.5933 6.74*
MA 12 0.6048 6.48*
Sudden-temporary Intervention ω0 0.0145 0.67
δ1 -0.6245 -0.77
Noise MA 1 0.6537 7.84*
MA 12 0.5916 5.95*
Sudden-permanent Intervention ω0 0.0020 0.08
Noise MA 1 0.6004 6.82*
MA 12 0.6020 6.51*
Teen multivariate
July 1998 intervention point
Gradual-permanent Intervention ω0 0.0001 0.00
δ1 0.5674 0.00
Covariate β0 0.4253 3.05*
Noise MA 1 0.5535 5.57*
MA 12 0.5534 5.83*
Sudden-temporary Intervention ω0 -0.0282 -0.82
δ1 0.1485 0.11
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
37
Table 10 (continued)
Intervention Model component Parameter Lag Estimate t
Covariate β0 0.4434 2.94*
Noise MA 1 0.4864 4.96*
MA 12 0.5134 5.19*
Sudden-permanent Intervention ω0 -0.0032 -0.09
Covariate β0 0.4115 2.98*
Noise MA 1 0.5271 5.18*
MA 12 0.5050 5.16*
January 1999 intervention point
Gradual-permanent Intervention ω0 0.0502 0.91
δ1 -0.0961 -0.12
Covariate β0 0.3746 2.85*
Noise MA 1 0.6331 6.68*
MA 12 0.4844 4.70*
January 98-June 98 Transition 0 0.0761 2.70*
Jul
y
98-December 98 Transition 0 0.0885 2.27*
Sudden-temporary Intervention ω0 0.0347 0.61
δ1 1.0342 36.67*
Covariate β0 0.3691 2.53*
Noise MA 1 0.5930 6.21*
MA 12 0.4932 4.77*
January 98-June 98 Transition 0 0.0723 2.39*
Jul
y
98-December 98 Transition 0 0.0792 1.83
Sudden-permanent Intervention ω0 0.0462 0.88
Covariate β0 0.3719 2.84*
Noise MA 1 0.6258 6.44*
MA 12 0.4807 4.75*
January 98-June 98 Transition 0 0.0763 2.65*
Jul
y
98-December 98 Transition 0 0.0863 2.17*
Note. All models included differencing at lags 1 and 12 to produce stationary residuals.
*p < .05, two-tailed.
The results of these involvement-based analyses are highly consistent with those based
on assigning crashes to the youngest involved driver. Specifically, the multivariate
results for total crash involvements indicated a statistically significant sudden-
temporary one-time 45.78% increase in teen crashes 6-months subsequent to the GDL
program implementation date. The results for fatal/injury crash involvements did not
indicate any sudden or gradual change in the fatal/injury crash involvements of the
teens at the implementation date or 6-months subsequent. The one difference between
these analyses and those based on the age of the youngest involved driver was that no
sudden-permanent decrease was found for the univariate adult total crash series. Since
the purpose of analyzing involvement rates was merely to determine whether the
findings would be consistent with the analyses of crashes categorized based on the age
of the youngest involved driver, the effects found for the crash involvement analyses
are not quantified here.
Total Crashes for 18-19-Year-Olds
One possible, though unintended, effect of implementing the GDL enhancements could
have been to increase the crash rates of 18-19-year-olds due to having the worst drivers
self-select themselves out of GDL by waiting until their 18th birthday to obtain a license.
At that point they would receive full licensing privileges without the potential benefits
of learning under conditions of reduced crash risk. In addition, some of the crash risk of
16-17-year-olds may have been shifted to 18-19-year-olds because these ‘older’ teen
drivers might have needed to transport their younger friends (instead of their younger
friends driving themselves) because of the nighttime and passenger restrictions. It is
also possible that the program’s delay of independent driving may have made 18-19-
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
38
year-olds less skillful as independent drivers than they otherwise would have been if
the program did not exist and they drove solo earlier. Alternatively, the GDL
enhancements could have had residual positive benefits that carried over to 18-19-year-
olds as the percentage of drivers in this age group licensed through the GDL program
became higher over time. A complete evaluation of the California’s GDL program
would therefore need to include some analyses of possible positive and negative effects
of the program enhancements on the crash rates for 18-19-year-olds.
Plots of total monthly crashes per 1,000 18-19-year-olds and 24-55-year-olds during the
period of January 1994 through December 2001 are shown in Figure 16. The average
total crash rate per 1,000 18-19-year-olds during this time period was 4.33 (SD = 0.35),
which is 2.24 times higher than the 15-17-year-old total crash rate of 1.93, and 3.08 times
higher than the adult per-capita rate of 1.41 for this same time period. The much higher
per capita total crash rate for 18-19-year-olds relative to 15-17-year-olds reflects the fact
that a higher percentage of teens in the former age group are licensed and are therefore
more likely to be involved in a crash than are persons in the latter group.
Figure 16. Monthly total crashes per 1,000 population for 18-19-year-olds and 24-55-year-
olds during January 1994 through December 2001 by age of youngest driver involved.
Table 11 presents the results of the time series analyses for the gradual-permanent,
sudden-temporary, and sudden-permanent intervention types for the 18-19-year-old
series, both with and without using the adult rate as a covariate. The analyses were
conducted using two different possible intervention points, July 1998 and January 1999,
in an attempt to remove any transition effects. The transition parameters were kept in
the models regardless of their level of statistical significance, while all other parameters
were evaluated using an alpha level of .05, two-tailed. The adult univariate rates are not
modeled here because they were modeled earlier (see Table 1).
0.0
1.0
2.0
3.0
4.0
5.0
6.0
TOTAL CRASHES
PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
Ages 24-55
Ages 18-19
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
39
Table 11
18-19-Year-Old Total Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter La
g
Estimate t
Teen univariate
Gradual-permanent Intervention ω0 0.0052 0.02
δ1 0.8099 0.14
Noise MA 1 0.5660 6.07*
MA 12 0.6748 8.15*
Sudden-temporary Intervention ω0 -0.0076 -0.05
δ1 1.0297 0.03
Noise MA 1 0.5051 5.13*
MA 12 0.5642 5.93*
Sudden-permanent Intervention ω0 -0.0381 -0.21
Noise MA 1 0.5675 6.29*
MA 12 0.6739 8.06*
Teen multivariate
July 1998 Intervention Point
Gradual-permanent Intervention ω0 0.0524 4.15*
δ1 0.9739 81.29*
Covariate β0 3.1340 15.54*
Noise MA 1 0.7816 10.10*
MA 12 0.6860 7.56*
Sudden-temporary Intervention ω0 -0.1566 -1.44
δ1 0.1628 0.23
Covariate β0 3.0891 13.01*
Noise MA 1 0.5490 5.55*
MA 12 0.5628 5.47*
Sudden-permanent Intervention ω0 -0.0122 -0.12
Covariate β0 3.0393 13.70*
Noise MA 1 0.5785 6.21*
MA 12 0.6100 6.25*
January 1999 Intervention Point
Gradual-permanent Intervention ω0 0.0885 2.91*
δ1 0.8908 19.64*
Covariate β0 3.0345 14.46*
Noise MA 1 0.6932 8.26*
MA 12 0.6270 6.58*
January 98-June 98 Transition 0 0.0982 1.40
Jul
y
98-December 98 Transition 0 0.1028 1.28
Sudden-temporary Intervention ω0 0.0032 0.07
δ1 1.1230 1.74
Covariate β0 3.0288 12.57*
Noise MA 1 0.5277 5.43*
MA 12 0.5627 5.10*
January 98-June 98 Transition 0 0.0767 0.96
Jul
y
98-December 98 Transition 0 -0.0139 -0.17
Sudden-permanent Intervention ω0 0.2696 1.55
Covariate β0 3.0597 13.86*
Noise MA 1 0.5902 6.70*
MA 12 0.5989 6.06*
January 98-June 98 Transition 0 0.1529 1.59
Jul
y
98-December 98 Transition 0 0.1432 1.09
Note. All models were differenced at lags 1 and 12 to produce stationary residuals.
*p < .05, two-tailed.
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
40
The multivariate gradual-permanent intervention model was statistically significant at
the actual July 1998 implementation date (ω = 0.0524, t = 4.15) and also 6-months
subsequent in January 1999 (ω = 0.0885, t = 2.91). However, the high δ parameter in the
July 1998 model is quite close to 1.00 (δ = 0.9739), suggesting that the effect does not
reach an asymptote during the time period analyzed (a so-called ‘ramp’ effect) and
indicates possible unreliability in the model parameter estimates. The July 1998 model
was therefore rejected. The January 1999 δ value was smaller (δ = 0.8908), indicating an
asymptote had been reached by the end of the time series period. The January 1999
gradual-permanent effect represents an increase of 0.0885 total crashes per 1,000 18-19-
year-olds the first month (a 2.12% increase over the 4.1667 January 1994 to December
1998 pre-intervention level), and stabilized at a post-intervention level of 0.8104 more
crashes per 1,000 capita (a 19.45% increase). Based on the January 1999 to December
2001 average monthly population of 973,173 18-19-year-olds, this translates into about
789 additional crashes per month or 9,464 additional crashes annually.
Fatal/Injury Crashes for 18-19-Year-Olds
Plots of fatal/injury monthly crashes per 1,000 18-19-year-olds and 24-55-year-olds
during the period of January 1994 through December 2001 are shown in Figure 17. The
average fatal/injury crash rate per 1,000 18-19-year-olds during this time period was
1.87 (SD = 0.17), which is 2.28 times higher than the 15-17-year-old fatal/injury crash
rate of 0.82, and 3.12 times higher than the adult fatal/injury per-capita rate of 0.60.
Again, the higher crash rate for 18-19-year-olds relative to younger teens probably
reflects the fact that a higher percentage of 18-19-year-olds are licensed and drive
(particularly unsupervised) than are 15-17-year-olds.
Figure 17. Monthly fatal/injury crashes per 1,000 population for 18-19-year-olds and 24-55-
year-olds during January 1994 through December 2001 by age of youngest driver involved.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
FATAL/INJURY CRASHES
PER 1,000 POPULATION
Jan-94
Jun-94
Nov-94
Apr-95
Sep-95
Feb-96
Jul-96
Dec-96
May-97
Oct-97
Mar-98
Aug-98
Jan-99
Jun-99
Nov-99
Apr-00
Sep-00
Feb-01
Jul-01
Dec-01
MONTH AND YEAR
Ages 24-55
Ages 18-19
GDL Enhancements
(July 1998)
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
41
The time series analysis results for the 18-19-year-old fatal/injury series are shown in
Table 12. The transition parameters were kept in the models regardless of their level of
statistical significance, while all other parameters were evaluated using an alpha level
of .05, two-tailed. None of the intervention parameters were statistically significant for
18-19-year-old fatal/injury crashes at the actual July 1998 implementation date or 6-
months subsequent, even after using the adult series as a covariate. This indicates that
the program implementation was not associated with a statistically significant sudden
or gradual change in the fatal/injury crash rates of 18-19-year-olds.
Table 12
18-19-Year-Old Fatal/Injury Crash Rate Model Statistics for Gradual-Permanent,
Sudden-Temporary, and Sudden-Permanent Interventions
Intervention Model component Parameter La
g
Estimate t
Teen univariate
Gradual-permanentaIntervention ω0 0.0220 1.45
δ1 0.9783 27.69*
Noise MA 1 0.6109 6.96*
MA 12 0.5467 5.78*
Sudden-temporaryaIntervention ω0 -0.0008 -0.00
δ1 -0.4605 -0.01
Noise MA 1 0.5462 5.55*
MA 12 0.4666 4.43*
Sudden-permanentaIntervention ω0 0.0342 0.43
Noise MA 1 0.5699 6.29*
MA 12 0.5443 5.76*
Teen multivariate
July 1998 Intervention Point
Gradual-permanentbIntervention ω0 0.0161 1.50
δ1 0.9674 27.03*
Covariate β0 3.2253 13.37*
Noise MA 1 0.5548 6.15*
MA 6 0.2764 2.64*
Sudden-temporarybIntervention ω0 -0.0461 -0.75
δ1 -0.3954 -0.31
Covariate β0 3.0906 12.73*
Noise MA 1 0.4875 5.34*
MA 6 0.2332 2.17*
Sudden-permanentbIntervention ω0 0.0139 0.21
Covariate β0 3.1874 12.90*
Noise MA 1 0.4780 5.23*
MA 6 0.2217 2.11*
January 1999 Intervention Point
Gradual-permanentbIntervention ω0 0.0547 1.58
δ1 0.8316 7.41*
Covariate β0 3.2504 13.35*
Noise MA 1 0.5751 6.43*
MA 6 0.2580 2.45*
January 98-June 98 Transition 0 0.0404 0.71
July 98-December 98 Transition 0 0.0590 0.93
Sudden-temporarybIntervention ω0 -0.1144 -1.43
δ1 0.7173 0.84
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
42
Table 12 (continued)
Intervention Model component Parameter La
g
Estimate t
Covariate β0 3.0514 12.17*
Noise MA 1 0.5080 5.50*
MA 6 0.2288 2.14*
January 98-June 98 Transition 0 -0.0279 -0.47
July 98-December 98 Transition 0 -0.0600 -0.91
Sudden-permanentbIntervention ω0 0.0750 0.79
Covariate β0 3.2472 12.71*
Noise MA 1 0.5003 5.62*
MA 6 0.2221 2.11*
January 98-June 98 Transition 0 0.0296 0.45
July 98-December 98 Transition 0 0.0301 0.37
aDifferenced at lags 1 and 12 to produce stationary residuals. bDifferenced at lag 1.
*p < .05, two-tailed.
DISCUSSION
This study analyzed several different crash types and age-groups, various intervention
models, and flexible intervention start points to determine whether the enhancements
made to the California teen licensing program in July 1998 resulted in crash reductions
for teen drivers. The results are summarized below:
• No overall reduction in total crashes or fatal/injury crashes was found immediately
following program implementation or beginning 6 months later. This outcome was
the same even when transition components were added to the models to adjust for
the influence of the influx of teen licensees before the implementation date, when
the adult series was included as a control variable, when only 16-year-old driver
crashes were analyzed, and when the rates were calculated as crash involvements
rather than being based on the youngest involved driver. However the program
was found to be associated with a 19.45% gradual-permanent increase in total
crashes for 18-19-year-olds 6 months after the program was implemented (about
9,464 additional crashes per year). No significant effect was found in the 18-19-year-
olds fatal/injury crashes.
• The 12-month nighttime restriction was associated with a sudden-permanent 0.44%
reduction in total crashes occurring during the hours of midnight to 5:00 a.m. for 15-
17-year-olds starting 1-year subsequent to the implementation of the nighttime
restriction. The results also suggested a marginally significant sudden-permanent
0.45% reduction in their nighttime fatal/injury crashes starting 1-year subsequent to
the program implementation. These effects translate into savings of 153 total crashes
and 68 fatal/injury crashes annually for 15-17-year-olds. These crash savings
estimates are based on an assumption that the GDL night driving restriction did not
increase daytime crashes.
• The 6-month passenger restriction was associated with a marginally significant
sudden-permanent 2.52% reduction in 15-17-year-old total teen passenger crashes,
and a significant gradual-permanent reduction stabilizing at -6.43% in fatal/injury
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
43
passenger crashes when using an intervention date 1-year subsequent to the
program start date. These effects equate to savings of 878 total crashes and 975
fatal/injury crashes annually for 15-17-year-olds. These crash savings estimates are
based on an assumption that the GDL passenger restriction did not cause an
increase in non-passenger crashes for the 15-17-year-old age group.
The fact that no overall reductions were found in teen total or fatal/injury crash rates
from the program start date or from a 6-months subsequent date is not surprising given
the Williams et al. (2002) findings indicating that many teens were applying for their
instruction permit earlier to avoid delaying licensure, and that only small increases
were found in the percentages of teens receiving additional hours and miles of
supervised on-the-road practice during this longer instruction permit period. In
addition, the reductions associated with the nighttime and passenger restrictions were
small and occurred some months later in time and therefore would not have helped
detect an effect using the time periods analyzed for the overall analyses.
The fact that an increase was found in total crashes for 18-19-year-olds suggests that
GDL programs may have unintended negative consequences for this and possibly other
age groups. One possibility for this finding is that any positive effects of the program
may not continue into later years and that 16-17-year-olds under the program might not
be as safe and skilled at age 18 as they would have been without the GDL restrictions.
The increase in 18-19-year-old crash rates could also be due to a higher percentage of
that age group being licensed due to younger teens waiting to license until age 18 to
avoid the program. In any case, it is recommended that 18-19-year-olds not be used as a
comparison group for evaluations of GDL programs because it appears that drivers in
this age group are impacted by such programs.
It should be noted that it is possible that the program impacted the crash rates for 24-55-
year-olds. This age group was selected as a control variable based on an assumption
that it would have been impacted by the same extraneous variables that impacted the
teen crash rates and also that it would not have been influenced by the program. It is
possible that the latter assumption may not be completely true. For example, it is
possible that the program may have resulted in more driving among adults due to their
needing to transport their teens for a longer period of time than before the program,
especially during late night hours. This could have elevated the crash rates for adults by
an unknown amount during the post period above what they would have been had the
program not been implemented. This would have tended to bias the analyses that
included the adult series as a control variable in favor of finding a positive impact of the
program on the crash rates of 15-17-year-olds. However any such biasing influence is
believed to be minimal. The analyses of the nighttime and passenger restrictions would
not have been subject to any such bias because the crash rates of adults were not used as
a control variable in those analyses.
Because the post-program crash rates for teens were compared to their pre-program
rates, and these pre-program rates already reflected the influence of crash reductions
associated with the original teen licensing program evaluated by Hagge and Marsh
(1988), any benefit of the program enhancements made in 1998 was expected to be only
marginal incremental reductions in crash rates. Indeed, the observed effects for the
nighttime and passenger restrictions were modest in size. If this evaluation had
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
44
compared the crash rates under this enhanced program with all of its components to
some theoretical set of teen crash rates for drivers under no program, it is much more
likely that significant and larger decreases in overall total and fatal/injury crash rates
associated with the program would have been found.
Finding reductions in total and fatal/injury nighttime crashes is consistent with results
from other states that have adopted nighttime restrictions (Ferguson et al., 1996;
McKnight et al., 1983). The use of a 1-year delayed intervention date for analyzing the
effects of the nighttime and passenger restrictions seems justified because it both
reduced transition bias associated with the increase in teen licensure around the time
the enhancements were implemented and allowed time for more teens to be fully under
the program requirements. This latter issue is especially relevant for evaluating the
restrictions because they do not take effect until the teens complete the 6-month
instruction permit period. The percentage reductions associated with the nighttime
restriction were, however, quite small. Larger nighttime crash reductions may have
been realized if the nighttime restriction began at an earlier time (e.g., 11:00 p.m. or
earlier), as has been suggested by traffic safety experts (McKnight, 1986; Williams &
Mayhew, 2003). In addition, although around 90% of teens complied for at least the first
6 months of the restriction, only 60% of teens were found to have not driven after
midnight for their first full year after licensure (Williams et al., 2002). Clearly if parents
could be motivated to not permit driving for the full term of the restriction, even larger
reductions in nighttime teen crashes might be realized.
California was the first state to implement a meaningful teen passenger restriction
(Williams et al., 2002). Finding that the passenger restriction was associated with
modest, but significant reductions in both total and fatal/injury crashes is noteworthy
because it indicates that passenger restrictions are effective components of GDL
programs. Although compliance with the 6-month passenger restriction was not found
to be very high (around 50%), not transporting other teenagers during the first 6-
months of driving represented the largest actual change in behavior before and after the
GDL enhancements were implemented (Williams et al., 2002). Therefore it is not
surprising that the effects of the passenger restriction were larger than those for the
nighttime restriction. Given the high crash risk of teen drivers when they transport
other teenagers, finding ways to increase the willingness of parents to enforce the
passenger restriction would likely result in additional crash savings.
Although the California GDL program evaluated in this report is considered to be one
of the strongest in the United States, there are additional features that could be added or
changed that may serve to strengthen the program even further. In addition to starting
the nighttime restriction at an earlier time and finding ways to increase compliance with
the nighttime and passenger restrictions, the program could be improved by making a
teen’s advancement from one stage of licensure to another contingent upon maintaining
a crash- and violation-free driving record, and by tying the passenger and nighttime
restrictions to the intermediate licensing stage rather than to a set period of time
(McKnight, 1986). Furthermore, compliance with the nighttime and passenger
restrictions could be increased by allowing law enforcement officers to stop teens
simply because they believe they are violating these restrictions (i.e., primary
enforcement).
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
45
Other authors (e.g., Mayhew & Simpson, 2002) have recommended that driver
education and training be integrated into GDL programs so that they are multi-staged,
with a basic driver education course before teens learn how to drive and an advanced
course after they have gained some experience driving on the road. More complex
topics, such as hazard perception, might be better taught in the advanced course where
experience on the road might make these topics more understandable. Results of a
recent evaluation (Masten & Chapman, 2003) showing that home-study driver
education courses were just as effective as classroom-based courses for teaching basic
driver education content may provide a means for removing some of the potential
roadblocks for integrating such a two-staged driver education and training system with
California’s GDL program. The use of home-study driver education for the first stage of
a tiered driver education and training program may also increase parental involvement
in their teen’s early driving experience, and motivate them to more fully enforce the
GDL restrictions.
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graduated licensing laws: The case of California. Available:
http://www.econ.ucalgary.ca/fac-files/fja/Draft_AAP_W981.pdf. Accessed March
25, 2003.
Bloch, S. A. (2000, September 14). California teen passenger deaths and injuries drop as
graduated driver license law marks second anniversary [News release]. Available:
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Bloch, S. A., Shin, H-C., & Labin, S. N. (2002, August 8). Does graduated driver licensing
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Cooper, P. J., Pinili, M., & Chen, W. (1995). An examination of the crash involvement
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on Operator Education and Regulation (Report No. 458), Transportation Research
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Ferguson, S. A., Leaf, W. A., Williams, A. F., & Preusser, D. F. (1996). Differences in
young driver crash involvement in states with varying licensure practices. Accident
Analysis and Prevention, 28, 171-180.
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APPENDIX
Provisional License for Minors: Distinctive Driver's License
12814.6. (a) ( )1 Except as provided in Section 12814.7, any driver's license issued to a
person at least 16 years of age but under 18 years of age shall be issued pursuant to the
provisional licensing program contained in this section. The program shall consist of all of the
following components:
(1) Upon application for an original license, the applicant shall be issued an instruction permit
pursuant to Section 12509. A person who has in his or her immediate possession a valid permit
issued pursuant to Section 12509 may operate a motor vehicle, other than a motorcycle or
motorized bicycle, ( )2 only when the person is either taking the driver training instruction
referred to in paragraph (3) or practicing that instruction, provided the person is
accompanied by, and is under the immediate supervision of, a California licensed driver
25 years of age or older whose driving privilege is not on probation. The age requirement of
this paragraph does not apply if the licensed driver is the parent, spouse, or guardian of the
permitholder or is a licensed or certified driving instructor.
(2) The person shall hold an instruction permit for not less than six months prior to applying for a
provisional driver's license.
(3) The person shall have complied with one of the following:
(A) Satisfactory completion of approved courses in automobile driver education and driver
training maintained pursuant to provisions of the Education Code in any secondary school of
California, or equivalent instruction in a secondary school of another state.
(B) Satisfactory completion of six hours or more of behind-the-wheel instruction by a driving
school or an independent driving instructor licensed under Chapter 1 (commencing with Section
11100) of Division 5 and either an accredited course in automobile driver education in any
secondary school of California pursuant to provisions of the Education Code or satisfactory
completion of equivalent professional instruction acceptable to the department. To be
acceptable to the department, the professional instruction shall meet minimum standards to be
prescribed by the department, which standards shall be at least equal to the requirements for
driver education and driver training contained in the rules and regulations adopted by the State
Board of Education pursuant to the Education Code. A person who has complied with this
subdivision shall not be required by the governing board of a school district to comply with
subparagraph (A) in order to graduate from high school.
(C) No student shall take driver training instruction, unless he or she is taking driver
education at the same time or has successfully completed driver education.
(4) The person shall complete 50 hours of supervised driving practice prior to the issuance of a
provisional license, which is in addition to any other driver training instruction required by law.
Not less than 10 of the required practice hours shall include driving during darkness, as defined
in Section 280. Upon application for a provisional license, the person shall submit to the
department the certification of a parent, spouse, guardian, or licensed or certified driving
instructor that the applicant has completed the required amount of driving practice and is
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
49
prepared to take the department's driving test. A person without a parent, spouse, guardian, or
who is an emancipated minor, may have a licensed driver 25 years of age or older or a licensed
or certified driving instructor complete the certification. This requirement does not apply to
motorcycle practice.
(5) The person shall successfully complete an examination required by the department. Before
retaking a test, the person shall wait for not less than one week after failure of the written test
and for not less than two weeks after failure of the driving test.
(b) ( )3 Except as provided in Section 12814.7, the provisional driver's license shall be subject
to all of the following restrictions:
(1) Except as specified in paragraph (3), during the first six months after issuance of a
provisional license the licensee shall not do any of the following unless accompanied and
supervised by a licensed driver who is the licensee's parent or guardian, a licensed driver who
is 25 years of age or older, or a licensed or certified driving instructor:
(A) Drive between the hours of ( )4 12 midnight and 5 a.m.
(B) Transport passengers who are under 20 years of age.
(2) During the second six months after issuance of a provisional license the licensee may
transport passengers under the age of 20 years between the hours of ( )5 5 a.m. and 12
midnight without supervision. This driving time restriction shall not modify or alter any local
ordinance that restricts or prohibits cruising during specified proscribed hours. However, the
restriction imposed under subparagraph (A) of paragraph (1) shall continue to apply during this
period.
(3) A licensee may drive between the hours of ( )4 12 midnight and 5 a.m. or transport an
immediate family member without being accompanied and supervised by a licensed driver who
is the licensee's parent or guardian, a licensed driver who is 25 years of age or older, or a
licensed or certified driving instructor, in the following circumstances:
(A) Medical necessity of the licensee when reasonable transportation facilities are inadequate
and operation of a vehicle by a minor is necessary. The licensee shall keep in his or her
possession a signed statement from a physician familiar with the condition, containing a
diagnosis and probable date when sufficient recovery will have been made to terminate the
necessity.
(B) Schooling or school-authorized activities of the licensee when reasonable transportation
facilities are inadequate and operation of a vehicle by a minor is necessary. The licensee shall
keep in his or her possession a signed statement from the school principal, dean, or school staff
member designated by the principal or dean, containing a probable date that the schooling or
school-authorized activity will have been completed.
(C) Employment necessity of the licensee when reasonable transportation facilities are
inadequate and operation of a vehicle by a minor is necessary. The licensee shall keep in his or
her possession a signed statement from the employer, verifying employment and containing a
probable date that the employment will have been completed.
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50
(D) Necessity of the licensee or the licensee's immediate family member when reasonable
transportation facilities are inadequate and operation of a vehicle by a minor is necessary to
transport the licensee or the licensee's immediate family member. The licensee shall keep in his
or her possession a signed statement from a parent or legal guardian verifying the reason and
containing a probable date that the necessity will have ceased.
(E) The licensee is an emancipated minor.
(c) A law enforcement officer shall not stop a vehicle for the sole purpose of determining
whether the driver is in violation of the restrictions imposed under subdivision (b).
(d) (1) Upon a finding that any licensee has violated paragraph (1) or (2) of subdivision (b), the
court shall impose one of the following:
(A) Not less than eight hours nor more than 16 hours of community service for a first offense
and not less than 16 hours nor more than 24 hours of community service for a second or
subsequent offense.
(B) A fine of not more than thirty-five dollars ($35) for a first offense and a fine of not more than
fifty dollars ($50) for a second or subsequent offense.
(2) If the court orders community service, the court shall retain jurisdiction until the hours of
community service have been completed.
(3) If the hours of community service have not been completed within 90 days, the court shall
impose a fine of not more than thirty-five dollars ($35) for a first offense and not more than fifty
dollars ($50) for a second or subsequent offense.
(e) No conviction of paragraph (1) or (2) of subdivision (b), when reported to the department,
shall be disclosed as otherwise specified in Section 1808 or constitute a violation point count
value pursuant to Section 12810.
(f) Any term of restriction or suspension of the driving privilege imposed on a person pursuant to
this subdivision shall remain in effect until the end of the term even though the person becomes
18 years of age before the term ends.
(1) The driving privilege shall be suspended when the record of the person shows one or more
notifications issued pursuant to Section 40509 or 40509.5. The suspension shall continue until
any notification issued pursuant to Section 40509 or 40509.5 has been cleared.
(2) A 30-day restriction shall be imposed when a driver's record shows a violation point count of
two or more points in 12 months, as determined in accordance with Section 12810. The
restriction shall require the licensee to be accompanied by a licensed parent, spouse, guardian,
or other licensed driver 25 years of age or older, except when operating a class M vehicle, or so
licensed, with no passengers aboard.
(3) A six-month suspension of the driving privilege and a one-year term of probation shall be
imposed whenever a licensee's record shows a violation point count of three or more points in
12 months, as determined in accordance with Section 12810. The terms and conditions of
probation shall include, but not be limited to, both of the following:
CALIFORNIA’S GRADUATED DRIVER LICENSING PROGRAM
51
(A) The person shall violate no law which, if resulting in conviction, is reportable to the
department under Section 1803.
(B) The person shall remain free from accident responsibility.
(g) Whenever action by the department under subdivision (f) arises as a result of a motor
vehicle accident, the person may, in writing and within 10 days, demand a hearing to present
evidence that he or she was not responsible for the accident upon which the action is based.
Whenever action by the department is based upon a conviction reportable to the department
under Section 1803, the person has no right to a hearing pursuant to Article 3 (commencing with
Section 14100) of Chapter 3.
(h) The department shall require any person whose driving privilege is suspended or revoked
pursuant to subdivision (f) to submit proof of financial responsibility as defined in Section 16430.
The proof of financial responsibility shall be filed on or before the date of reinstatement following
the suspension or revocation. The proof of financial responsibility shall be maintained with the
department for three years following the date of reinstatement.
(i) Notwithstanding any other provision of this code, the department may issue a distinctive
driver's license, which displays a distinctive color or a distinctively colored stripe or other
distinguishing characteristic, to persons at least 16 years of age and older but under 18 years of
age, and to persons 18 years of age and older but under 21 years of age, so that the distinctive
license feature is immediately recognizable. The features shall clearly differentiate between
drivers' licenses issued to persons at least 16 years of age or older but under 18 years of age
and to persons 18 years of age or older but under 21 years of age.
If changes in the format or appearance of drivers' licenses are adopted pursuant to this
subdivision, those changes may be implemented under any new contract for the production of (
)6 drivers' licenses entered into after the adoption of those changes.
(j) The department shall include, on the face of the provisional driver's license, the original
issuance date of the provisional driver's license in addition to any other issuance date.
(k) This section shall be known and may be cited as the Brady-Jared Teen Driver Safety Act of
1997.
Amended Sec. 8, Ch. 760, Stats. 1997. Effective January 1, 1998.
Amended Sec. 19, Ch. 1035, Stats. 2000. Effective January 1, 2001.
Amended Sec. 13.5, Ch. 758, Stats. 2002. Effective January 1, 2003.
The 2002 amendment added the italicized material, and at the point(s) indicated, deleted the
following:
1. "Notwithstanding any other provision of law"
2. "subject to Section 12509 only if that person is accompanied by, and under the immediate
supervision of, a driver who is 25 years of age or older, who holds a driver's license issued
under this code, and "
3. "Commencing July 1, 1998,"
4. "12:00 a.m."
5. "12:00 a.m."
6. "driver's"
