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ROUNDABOUT SAFETY - INFLUENCE OF SPEED, VISIBILITY AND DESIGN

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Roundabout design in New Zealand generally follows the Austroad guideline for intersection design (Austroads, 2005), which recommends long approach sight distances and provision of relatively high design speeds. This is in contrast to European based design philosophy where visibility is normally restricted and the geometric design encourages slow approach and negotiation speeds. This paper reports on the results of a study that used crash prediction models to investigate how the characteristics of roundabouts influences safety at 104 roundabouts in three centres. Using a dataset that contains pedestrian, cyclist and motor vehicle flows, approach and circulating speeds and sight distances an analysis was carried out for a number of crash types and new crash relationships established. It will be shown that safety benefits can be achieved by a more European based design philosophy.
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ROUNDABOUT SAFETYINFLUENCE OF SPEED, VISIBILITY AND
DESIGN
Dr Shane Turner, Associate (Transport), Beca Infrastructure Ltd
shane.turner@beca.com
Aaron Roozenburg, Transportation Engineer, Beca Infrastructure Ltd
aaron.roozenburg@beca.com
ABSTRACT
Roundabout design in New Zealand generally follows the Austroad guideline for intersection design
(Austroads, 2005), which recommends long approach sight distances and provision of relatively
high design speeds. This is in contrast to European based design philosophy where visibility is
normally restricted and the geometric design encourages slow approach and negotiation speeds.
This paper reports on the results of a study that used crash prediction models to investigate how the
characteristics of roundabouts influences safety at 104 roundabouts in three centres. Using a
dataset that contains pedestrian, cyclist and motor vehicle flows, approach and circulating speeds
and sight distances an analysis was carried out for a number of crash types and new crash
relationships established. It will be shown that safety benefits can be achieved by a more European
based design philosophy.
Introduction
New Zealand has a large number of roundabouts, which have been installed over a number of
decades. Over the decades roundabout design standards have evolved considerably, and later
designs are generally of higher standard. Many older roundabouts were installed prior to the
widespread introduction of safety audits and therefore a number of deficiencies, which would be
identified in more recent times, were not picked-up. There are a number of roundabouts still in
operation that have fundamental deficiencies including inadequate deflection, two approach lanes
but only one circulating lane, inconsistent visibility on different approaches, which leads to
differential negotiation speeds, and poor camber/superelevation, which can cause problems for
trucks.
Despite their many faults, the majority of roundabouts have relatively good safety records,
compared with other intersection control types (signals, priority controlled and uncontrolled),
particularly in high-speed environments. In many parts of the country roundabouts are a preferred
intersection treatment, as up to relatively high traffic volumes they have the benefit of keeping the
traffic flowing, particularly outside peak periods, when compared with signalised intersection
control, with the lost-time such intersection experience. They are however unpopular, particularly
larger roundabouts, with cyclists and pedestrians, with crash occurrence for the former being a
higher proportion of all intersection accidents when compared with other forms of control.
While there has been a lot of research/discussion on the safety of roundabouts, which has resulted in
changes to the design standards and a list of matters to consider in safety audits, there have been
few studies that have tried to quantify the effect of deficiencies on roundabouts safety and in
particular the safety of cyclists and pedestrians. The paper contains research that quantifies,
through the use of accident prediction models the effect of a number of variables on roundabout
safety. This research can be used by safety auditors, and other transport professionals, to estimate
the impact on safety of a particular deficiency and prioritise intersections for treatment.
The research presented in this paper focuses on the relationship between accidents, speed, traffic
volume and sight distance for various approach and circulating movements at roundabouts. The
flow-only models previously developed by Turner (2000) have been extended to include observed
speed, sight distance and intersection layout variables in various forms. Given the different impact
vehicle speed is expected to have on theactivemodes (walking and cycling), separate models
have been developed for the major accident type for each mode.
Selected Sites
In total a sample set of 104 roundabouts were selected in Auckland, Christchurch and Palmerston
North. The sites were selected so that a variety of different layouts and sizes were included in the
sample from around the country. Table 1 shows a breakdown of the sites by location and
roundabout type.
Table 1: Roundabout Locations and Types
Type Location
Christchurch
Auckland Palmerston
North Total
Single Lane Circulating
3-arm 0 2 2 4
4-arm 35 22 8 65
Two Lane Circulating
3-arm 0 4 0 4
4-arm 4 21 3 28
5-arm 0 3 0 3
TOTAL 39 52 13 104
A smaller sample set of 17 high-speed roundabouts was also selected from around the country.
This included sites in Christchurch, Auckland, Hamilton and Tauranga. A high-speed roundabout
must have one road that has a speed limit of 80km/h or more. Given the limited number of sites
that meet these criteria, all high-speed roundabouts for which data was readily available were
included in the sample set.
Predictor variables
A list of key roundabout variables was specified following the outcomes of a workshop with experts
in roundabout design, a review of overseas studies on roundabouts and a review of the publication
Ins and Outs of Roundabouts (Transfund 2000). This process identified the following variables:
sight distance, approach and negotiation speed, traffic and cyclist volume, deflection, approach and
exit curve design, approach and circulating road width.
Traffic, Pedestrian and Cycle Volume Data
The flow variables used in the urban roundabout intersection models are versions of those defined
in Turner (1995), where each movement is numbered in a clockwise direction starting at the
northern-most approach. Approaches are also numbered using the same technique and are
numbered in a clockwise direction (see Figure 1).
Individual movements are denoted as a lower case character for the user type (e.g. qi). Totals of
various movements are denoted with an upper case character (e.g. Qi). Models are developed for
each approach and are defined using the totals of various movements. These are:
Qe Entering volume for each approach.
Qc Circulating flow perpendicular to the entering flow.
Qa Approach flow (two-way flow on intersection leg).
Three one-hour manual turning volume counts were collected at each site, in the morning, evening
and at mid-day. Weekly, daily and hourly correction factors from theGuide to Estimation and
Monitoring of Traffic Counting and Traffic Growth (TDG, 2001) were used to estimate the AADT.
Visibility and Speed
Speeds measured in this study are the free speeds of vehicles travelling through the roundabouts and
not of vehicles turning left, right or having to give way. The visibility and speed variables used in
the models are shown in Table 2. Diagrams of vehicle speeds and the measurement of visibility
variable can be found in Figure 2 and Figure 3 respectively.
Table 2: Visibility and Speed Variables
Variable Description
VLL visibility from the limit line to vehicles turning right or traveling through the
roundabout from the approach to the right;
V10 visibility from 10 metres back from the limit line to vehicles turning right or
traveling through the roundabout from the approach to the right;
V40 visibility from 40 metres back from the limit line to vehicles turning right or
traveling through the roundabout from the approach to the right;
SLL free mean speed of entering vehicles traveling through the roundabout at the
limit line;
SC free mean speed of circulating vehicles traveling through the roundabout as they
pass the approach being modeled;
SSDLL standard deviation of free speeds of entering vehicles at the limit line;
SSDC standard deviation of free speeds of circulating vehicles as they pass the
approach being modeled;
Intersection Layout
Data on the layout of each roundabout were collected on site. From this data, variables were
developed to represent different situations; these variables were not of the continuous type such as
vehicle flows and mean speeds; and were incorporated into the accident prediction models as
covariates. The covariates are represented by multiplicative factors that are used to adjust the
prediction if the feature is present. The covariates used in the modelling process and their
definitions are shown in Table 3.
Table 3: Intersection Layout Covariates
Variable Description
ФMEL Multiple entering lanes
ФMCL Multiple circulating lanes
Ф3ARM Intersections with three arms
ФGRADD Downhill gradient on approach to intersection
Research is currently underway to consider other layout variables; continuous variables; including
approach, circulating and exiting curve radius, distance to upstream approach, total width of
approach and deflection. These variables are to be incorporated into future accident prediction
models and negotiation speed models.
Accident Data
Accident data for roundabouts nationally was extracted from the Ministry of Transports Crash
Analysis System (CAS) for the period 1January 2001 to 31 December 2005. During this period
there were 1202 reported injury accidents at urban roundabouts in New Zealand, including 7 fatal
and 154 serious accidents. This compares to the 365 reported injury accidents, including 2 fatal and
44 serious accidents, that occurred at the 104 urban roundabouts included in the sample set.
Modelling results
The models were developed using generalised linear modelling methods. Generalised linear models
were first introduced into road safety by Maycock and Hall (1984), and extensively developed in
Hauer et al. (1988). These models were further developed and fitted using accident data and traffic
counts in the New Zealand context for motor vehicles only accidents by Turner (1995).
The aim of the models is to develop relationships between flows, non-flow contributing variables
(the independent variables) and the mean number of accidents (the dependant variable).
The typical mean-annual numbers of reported injury accidents for urban roundabouts can be
calculated using turning movement counts, non-flow data and the accident prediction models in
Table 4. The total number of accidents can be predicted by summing the individual predictions for
each accident group on each approach.
The flow variables used in these models are for daily average flows and are shown graphically in
Figure 1. Table 2, Figure 2 and Figure 3 define the visibility and speed variables.
Table 4: Urban roundabout accident prediction models
Accident Type Equation (accidents per approach) Error
Structure GOF**
Entering-vs-
Circulating (Motor-
vehicle only) 13.2
26.047.0-8
11012.6
C
ceUMAR
S
QQA
××××= NB
(k=1.3)* 0.26
Rear-end (Motor-
vehicle only) e
Q
eUMAR eQA 42.2
38.0-2
21063.9 ×××= NB
(k=0.7)* 0.25
Loss-of-control
(Motor-vehicle only)
68.0
10
59.0-6
31036.6 VQA aUMAR ×××= NB
(k=3.9)* 0.25
Other (Motor-
vehicle only) MELaUMAR QA φ×××= 71.0-5
41034.1
66.2=
MEL
φ
Poisson 0.17
Pedestrian a
Q
UPAR ePA 67.060.0-4
11045.3 ×××= NB
(k=1.0)* 0.17
Entering-vs-
Circulating (Cyclist
circulating) 49.0
38.043.0-5
11088.3
LL
ceUCAR
S
CQA
××××= NB
(k=1.2)* 0.61
Other (Cyclist) 23.004.1-7
21007.2 aaUCAR CQA ×××= Poisson 0.50
Accident Type Equation (accidents per approach) Error
Structure GOF**
All Accidents MELaUAAR QA φ×××= 58.0-4
01011.6
66.1=
MEL
φ
NB
(k=2.2)* 0.28
*k is the gamma distribution shape parameter for the negative binomial (NB) distribution.
**GOF (Goodness Of Fit statistic) indicates the fit of the model to the data. A value of less than
0.05 indicates a poor fit whereas a high value indicates a very good fit.
Effect of Higher Speed Limits
Using the link data collected from the high-speed roundabouts with speed limits greater than
70km/h, a covariate analysis of the effect of higher speed limits on accidents was carried out. The
following model was developed using a data set that contains approach flows, accidents on each
approach and the respective speed limit grouping:
HSaRAXR QA φ×××= 66.04
1021.3 35.1=
HS
φ
The model is a good fit and has a negative binomial dispersion parameter (k) of 1.9. The covariate
for the higher speed sites indicates that at speed limits of 80 km/hr or greater there are 35% more
reported injury accidents than at a roundabout with an urban speed limit, for a given traffic volume.
Discussion
The majority of the preferred models for motor-vehicle and pedestrians accidents include non-flow
variables. This justifies the extension of previousflow-only models to include the non-flow
variables
For the motor-vehicle entering versus circulating accidents, the non-flow variable is the mean speed
of circulating vehicles (Sc). The exponent on this variable indicates that as circulating speeds
increase so does the number of accidents. For example, the model suggests that if mean circulating
speeds of 26 km/hr were reduced by 20% then the resulting reduction in accidents of this type
would be 38%. Figure 4 shows the change in accident numbers as the speed increases (for entering
volume of 5000 vpd and circulating of 6000 vpd). The accident rate for a circulating speed of
60km/h is almost 10 times that of a circulating speed of 20km/h.
The total accident model for high-speed roundabouts also shows that as the speed limit increases
the number of accidents increase. Roundabout with a speed limit above 70km/h have on average
35% more reported injury accidents than those below 70km/h.
The research implies that the European approach to the design of roundabouts, of lower speed, has
merit from a safety perspective.
Examination of the correlation matrix indicates that the speed of circulating vehicles is correlated to
the flow of circulating vehicles. This may be a result of roundabouts at higher volumes being
designed for faster speeds, for capacity reasons. There is therefore a clear capacity-safety trade-off.
The loss-of-control model was the only preferred model to include a visibility variable. In
developing models for other accident types the only other model where it featured as a stronger
predictor variable than speed was for other cyclist accidents. The exponents of the visibility
variables were consistent, however, taking positive values ranging from 0.08 to 0.8 for most
accident types except both other accident types (other cyclist, and other motor-vehicle) where they
were generally in the range -0.3 to -0.4. The reason for most accident types showing an increase in
accidents with increased visibility is likely to be the result of associated speed increases. It is
unclear why this would be different for otheraccidents.
For the other motor-vehicle and all accident models the preferred models included the covariate
for number of entering lanes. Both these models indicate that the accident rate is higher if the
roundabout has multiple entry lanes for a given traffic flow. No matter which accident type was
being modelled, every time this variable was included the covariate was always greater than 1.0.
This strong result indicates the reduced safety of multi-lane roundabouts when compared to single
lane roundabouts.
The models developed can be compared with those of previous studies, as illustrated here by
comparing models developed for entering-versus-circulating accidents developed in Turner (2000)
and Turner et al. (2006b). To allow for this comparison, the flow only models developed for this
study are shown in Table 5 along with the model for cyclist circulating accidents from Turner et al.
(2006b) and the model for accidents involving all wheeled road users (eg. includes motor-vehicles
accidents only and those with cyclists) in Turner (2000).
Table 5 shows that the relationships between the flow variables and motor-vehicle accidents are
similar for the current study and the Turner (2000) study. The higher coefficient for the earlier
study is likely to be the result of a downward trend in accidents in New Zealand and the inclusion of
cyclist accidents. It is interesting that the models for cyclist accidents have similar exponents on the
circulating flow variable to the models for motor-vehicle only accidents. This indicates that similar
relationships between flows and accidents may exist for both road user groups.
Table 5: Entering-versus-circulating accident prediction models
Model Study Equation (accidents per approach)
Motor Vehicle Only
Accidents Current
Study 37.048.0-5
11049.2 ceUMAR QQA ×××=
Motor Vehicle (only and
with cyclists) Accidents Turner 2000 41.042.0-4
11014.1 ceUWXR QQA ×××=
Cyclist Circulating
Accidents Current
Study 38.046.0-4
11051.1 ceUCAR CQA ×××=
Cyclist Circulating
Accidents Turner et al.
(2006b) 32.079.0-5
11040.2 ceUCXR CQA ×××=
Summary
This paper presents a number of accident prediction models that have been developed for
roundabouts in urban and rural road networks. Models have been developed for the major accident
types for motor vehicles only, motor vehicles versus cyclists and pedestrians versus motor vehicle
classifications. The models include the principal flow variables and a number of non-flow
variables. Multiplicative factors have been produced to show the difference in accident rate for low
speed (70 km/hr and less) and high speed (80 km/hr and more) at roundabouts.
The preferrednon-flow models include a number of the variables that were collected in addition
to the flow variables, including visibility, speed and multiple entry lanes. While not in all preferred
models, there were strong relationships observed between visibility and number of entry lanes with
accident occurrence.
The models indicate that there would be benefits in a move to European design standards that
reduce both circulating and entry speeds. For example, the models indicate that reduction of mean
circulating free speeds of 26km/hr by 20% would result in a 38% accident reduction in entering-
versus circulating accidents. The models also predict that the entering versus circulating accident
rate is 10 times worse at a circulating speed of 60km/h, compared with a circulating speed of
20km/h.
There are also benefits possible through reduction of visibilities. Further research, however, is
required to explain why the models indicate that other motor-vehicle accidents may increase with
reduced visibilities. Research is currently underway to look at the effects on safety of more
geometric features, continuous features, including deflection, entering an exiting radius and
approach width.
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Lund, Sweden (1997).
G. Maycock and R.D.Hall, Accidents at 4-arm roundabouts Laboratory Report, LR1120,
Crowthorne, Berks, U.K Transport Research Laboratory (1984).
Traffic Design Group (TDG), Guide to Estimation and Monitoring of Traffic Counting and Traffic
Growth, Transfund Research Report No. 205, Transfund New Zealand, Wellington, New Zealand.
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Cyclists Accidents, Towards Sustainable Land Transport Conference, Wellington, NZ. (2004).
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Pedestrians, Land Transport NZ Research Report (to be published late 2006), Land Transport NZ,
Wellington NZ, (2006b)
S. A. Turner, G. R. Wood and A. P. Roozenburg, Rural Intersection Accident Prediction Models,
22nd ARRB Conference, Melbourne, Australia. (2006a)
G. R. Wood, Generalised Linear Accident Models and Goodness-of-fit Testing, Accident
Analysis and Prevention 34 (2002) pp. 417 427.
Figures
North
65
43
12
Approach 1
Approach 2
Approach 3
North
6
54
3
12
Approach 1
Approach 2
Approach 3
Approach 4
8
9
12
11
10
7
1
3
2
4
5
6
78
20
19
18
1
7
10
9
11
12
15
14
13
16
North
Approach 1
Approach 2
Approach 3Approach 4
Approach 5
Figure 1: Numbering convention for movements and approaches
Figure 2: Measuring points for entering and circulating vehicle speeds
Figure 3: Measurement of V10
Vehicle
Speed
Circulatin
g Vehicle
Speed
0.000
0.050
0.100
0.150
0.200
0.250
0 10 20 30 40 50 60 70
Mean Circulating Speed
E Vs C Accidents per year by
approach
Figure 4: Effect of Circulating Speed on Entering Versus Circulating Accidents
(Qe of 5000 and Qc of 6000)
... Further research in New Zealand (Turner and Roozenburg 2006) revealed that roundabouts with approaching speeds higher than 44 mph had 35% more injury crashes than those with lower speeds. It is unclear if these roundabouts with high-speed approaches represented both rural and urban environments and both single-lane and multilane roundabouts. ...
... New Zealand (Thomas and Nicholson 2003) Rural Lower level of alertness in rural roundabouts requires supplementary measures on the approach to warn drivers in advance. New Zealand (Turner & Roozenburg 2006) High-speed (45mph+) High-speed roundabouts have more injury crashes than those with lower speeds. Belgium (Antoine 2005) Including rural, suburban An average of 42% decrease in injury crashes and 48% decrease in serious crashes in all settings. ...
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When the functional form of a multivariate statistical model does not match the phenomenon it aims to describe, or when important explanatory variables are missing, then well estimated regression constants can not be trusted to predict what will be the effect on the dependent variable of a change in an explanatory variable. We have good tools to estimate regression constants but insufficient tools to determine what functional form fits a phenomenon and what explanatory variables need to be introduced into the model equation. The aim of this paper is to suggest two tools capable of guiding the choice of an appropriate functional form for the model equation and whether a candidate explanatory variable promises to be useful. Both tools are based on the idea that cumulative (integral) functions may reveal order where atomic presentations of the same data fail. The example to which these tools are applied is the modeling of accident frequency for two-lane rural roads in Maine. While the example is specific, the domain of application of the tools is general.
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