Conference PaperPDF Available

Roundabout crash prediction models June 2009

Authors:
ACCIDENT PREDICTION MODELS FOR
ROUNDABOUTS
Dr Shane Turner, Beca Infrastructure Ltd, New Zealand
Professor Graham Wood, Macquarie University, NSW, Australia
Aaron Roozenburg, Beca Infrastructure Ltd, New Zealand
ABSTRACT
The management of speed is considered an important safety issue at roundabouts. The
approach speed and negotiating speed through roundabouts depends on both the geometric
design of the roundabout and sight distance. In New Zealand and in the Australian States the
design standards (based on Austroads) recommend long approach sight distance and provision
of relatively high design speeds. This is in contrast to European roundabouts where visibility is
normally restricted and the geometric design encourages slow approach and negotiation
speeds. The ‘flow-only’ models developed in Turner (2000) have been extended to include sight
distance, intersection layout and observed speed variables. Models have been produced for the
major motor-vehicles only, pedestrian versus motor-vehicles and cyclists versus motor-vehicle
accident types. ‘Flow-only’ models have also been produced for roundabouts on roads with
high speed limits. The models have then been used to assess whether a move towards the
European design philosophy, of lower speed and reduced sight distance, is likely to produce
lower accident rates for all modes, particularly in areas that have high concentrations of
pedestrians and cyclists.
INTRODUCTION
Safety deficiencies in existing and proposed roundabouts have received considerable attention
from safety auditors over the last 10 years or more. The reoccurrence of common deficiencies
in the design of new roundabouts in New Zealand has lead to the publication of the guide “The
Ins and Outs of Roundabouts”, which was published by Transfund NZ (2000). This guide
provides a list of problems that have been encountered in 50 safety audit reports. Visibility and
geometric design features, particularly inadequate deflection and marking, feature as problems
in many of the safety audit reports. While not specifically mentioned in this report, approach
and negotiating speed do have the potential to exacerbate any sight distance, geometric and
other deficiencies present at a roundabout. As far as we are aware the effect on safety of speed
at roundabouts, in combination with other deficiencies, has not previously been investigated
using accident prediction modelling methods in New Zealand. The models enable the combined
impact of speed and each of the other key variables on accident risk to be quantified. Safety
auditors and other transport professionals can use the models to assess the impact on safety of
a particular deficiency and prioritise for treatment.
Roundabouts, particularly large and 2-lane roundabouts, have a poor safety record with respect
to cyclists. This is illustrated in the proportion of injury accidents involving cyclists at
roundabouts (25%), compared with traffic signals (7%) and priority junctions (16%). Many cycle
advocates have strong opinions on this matter and strongly oppose the use of roundabouts,
particularly larger roundabouts on cycle routes. There are two reasons given for this increased
accident risk to cyclists; 1) as roundabouts become larger, with more lanes, and often higher
speeds, they become more complex to negotiate by motor-vehicle drivers, and motorists are
more likely to not see the cyclist, due to their relatively small size and 2) as motor-vehicle
speeds increase, the relative speed between cyclists and motor-vehicles increases and there is
more potential that drivers will overtake cyclists in an unsafe manner or that cyclists will
misjudge the gap/space required for various manoeuvres. So it is expected that reduced
vehicle speeds and complexity (single lane circulating) should improve safety for cyclists.
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 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 the ‘active’ modes (walking and cycling),
separate models have been developed for the major accident type for each mode.
SITE SELECTION
The research team had access to an existing sample set of roundabouts that was collected in
two previous studies, by Turner (2000) and Turner et al. (2006b). The majority of the sites in the
more recent study were in Christchurch, and were single lane, 4-arm intersections. A number of
additional sites were added from Auckland and New Plymouth to increase the sample size and
enable other roundabout types to be added.
Site Selection Criteria
A roundabout is made up of a series of give-way controlled T-junctions, where the through (or
circulating) route is one-way. Roundabouts can be large or small and can have one or more
circulating and entry lanes. There is significant variety in the roundabouts found in New
Zealand due to different design practices through many decades.
The most common roundabout in New Zealand has four-arms with one circulating lane.
Previous studies on roundabouts by Turner (2000) and Turner et al. (2006b) concentrated on
this common roundabout type. Even within this roundabout type there is a lot of variety in terms
of central island diameter, approach design and overall roundabout shape.
The research steering group and research team were of the view that a broader sample of
roundabouts should be included in this study, so that the effects of speed, visibility and layout
could be better assessed. Hence the sample set for this study includes 3, 4 and 5 arm
roundabouts, with different distances between arms. The sample includes both single and dual
entering and circulating lanes, although there are considerably more roundabouts with single
entering and circulating lanes.
While a wide variety of roundabout types were included in the sample set, sites that had been
constructed within the last five years or had undergone significant modification during this period
were excluded.
Sample Size
Experience in other studies of this type indicates that a sample set of at least 100 sites is
necessary to developed accident prediction models for the major accident types. A large
sample size is particularly important in this study, where there is a variety of intersection types, a
lot of non-flow variables and when looking at accidents involving less common modes, such as
cyclists and pedestrians.
In total a sample set of 104 roundabouts were selected in Auckland, Christchurch and
Palmerston North. 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
There is no database that lists all the roundabouts in New Zealand. So it was not possible to
use a formal sampling procedure to select a sample of sites that meet the criteria. The sites
were instead selected so that a variety of different layouts and sizes were included in the
sample from around the country.
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 & DATA COLLECTION
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. The following
sections describe the data that was collected.
Traffic 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 (the sum of the entering and exiting flow for each approach).
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 the “Guide to
Estimation and Monitoring of Traffic Counting and Traffic Growth” (TDG, 2001) were used to
estimate the AADT. The hourly factors were calculated from flow profiles for the different road
types.
For the analysis of high-speed roundabouts, approach volumes (Qa) have been used. The
volumes for the high-speed intersections have been estimated from the link volumes collected
through tube counting programmes.
Pedestrian and Cyclist Volume Data
Manual pedestrian and cyclist movement counts were collected at each site, in the morning,
evening and at mid-day. Like motor-vehicle counts, seasonal, daily and hourly correction factors
were used to estimate annual averaged daily volumes. These factors were calculated from
continuous counts collected for a previous study (Turner et al., 2006b).
Cyclist flow variables used in the urban roundabout intersection models are defined in an
identical process to motor vehicle movements, numbered in a clockwise direction at
intersections, starting at the northern-most approach. Individual cyclist movements are denoted
as a lower case character for the user type (e.g. ci) and totals of various movements are
denoted with an upper case character (e.g. Ci).
Pedestrian flow variables used in the models are defined as the number of pedestrians crossing
each approach in either direction. The total crossing volume for each approach is denoted as
an upper case P.
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
Accident Data
Accident data for roundabouts nationally was extracted from the Ministry of Transport’s 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, including 7 fatal and 154
serious accidents. This compares to 365 reported injury accidents, including 2 fatal and 44
serious accidents at the 104 urban roundabouts included in the sample set.
Figure 4 shows the proportion of reported injury accidents at roundabouts nationally involving
single motor-vehicles only, multiple motor-vehicles only, cyclists, and pedestrians. This shows
that 67% (around ) of accidents involve motor vehicles only and 25% involve a cyclist.
Figure 5 shows the proportion of major injury accident types occurring nationally; this has not
been categorised by vehicle type. However, the most common cycle accident type is entering
versus circulating (82% of cycle accidents), 74% of which occur with the cyclist circulating and a
motor vehicle entering.
Based on this analysis, the accident type groups to be modelled were determined. All accident
types were grouped by road user involvement and proportion of accident type. The accident
groups are as follows:
Entering-vs-Circulating (Motor-vehicle only)
Rear-end (Motor-vehicle only)
Loss-of-control (Motor-vehicle only)
Other (Motor-vehicle only)
Pedestrian
Entering-vs-Circulating (Cyclist circulating)
Other (Cyclist)
MODELLING RESULTS
Using the process outlined in the accompanying conference paper (Turner et al, 2006a),
accident prediction models were developed for the main accident types at roundabouts for each
mode. A number of models were developed that incorporated flow and non-flow variables. The
Bayesian Information Criterion (BIC) and a goodness-of-fit measure for these models were then
calculated, in order to determine the ‘preferred model’ for each accident type. The preferred
model includes all conflicting flows, fits the accident data well and contains a parsimonious set
of variables.
Model Interpretation
This section outlines the interpretation of model parameters and how these relate predictor
variables to accidents. Caution should always be exercised when interpreting relationships as
in some multiple predictor variable models two or more variables can be highly correlated. If
this occurs then the exponents can be difficult to interpret. The modelling process described in
the accompanying paper (Turner et al. 2006a), however, often means that variables in the
‘preferred’ model are not highly correlated. This is because the method acknowledges that
adding a variable correlated to those already in an existing model does not improve the fit of the
model compared to the addition of important non-correlated variables. Hence, interpretation of
model parameters can be straightforward for the preferred models (those presented in this
paper).
A typical model describes the relationship between the mean number of accidents and
predictors such as traffic volumes and non-flow variables. For two continuous variables (such
as flows or speeds) the model usually takes the form of a power function:
21 210 bb xxbA
,
where A is the annual mean number of accidents, x1 and x2 are average daily flows of vehicles
or non-flow variables, and b1, b2 and b3 are model parameters.
In this model form the parameter b0 acts as a constant multiplicative value. If the number of
reported injury accidents is not dependent on the values of the two-predictor variables (x1 and
x2), then the model parameters b1 and b2 are zero. In this situation the value of b0 is equal to
the mean number of accidents. The value of the parameters b1 and b2 indicate the relationship
that a particular predictor variable has (over its flow range) with accident occurrence. There are
five types of relationship for this model form, as presented in Figure 6 and discussed in Table 4.
Table 4: Relationship between predictor variable and accident rate
Value of Exponent
Relationship with accident rate
bi > 1
For increasing values of the variable, the number of accidents will
increase, at an increasing rate
bi = 1
For increasing values of the variable, the number of accidents will
increase, at a constant (or linear) rate
0 < bi < 1
For increasing values of the variable, the number of accidents will
increase, at a decreasing rate
bi = 0
There will be no change in the number of accidents with increasing
values of the variable
bi < 0
For increasing values of the variable, the number of accidents will
decrease
Generally, accident prediction models of this form have exponents between bi = 0 and bi = 1,
with most flow variables having an exponent close to 0.5, i.e. the square root of flow. In some
situations, however, parameters have a value outside this range.
In the case of models including a covariate (here, discrete variables with a small number of
alternatives) a multiplier for different values of the variable is produced, and it is easy to interpret
the relationship. This factor indicates how much higher (or lower) the number of accidents is if
the feature is present. A factor of 1 indicates no effect on accident occurrence.
It is important to note that these relationships apply only to models of the above form (power
function models). Other model forms are tested in the modelling process (for example
polynomials, Hoerl’s function and combination power and exponential functions). The
interpretation of these is often more complex and to examine the relationship between predictor
variables and accidents the model should always be plotted.
Preferred Models
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 5. 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 5: 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
71.0-5
41034.1
66.2
MEL
Poisson
0.17
Pedestrian
a
Q
UPAR ePA 67.0
60.0-4
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
All Accidents
MELaUAAR QA
58.0-4
01011.6
66.1
MEL
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.
Discussion
Unlike previous models developed by the study team, two of the seven preferred models had
model forms that deviated from the traditional ‘power function’ form. The data for these accident
types (pedestrian versus motor-vehicle and rear-end motor-vehicle) exhibit relationships
between the flow variables and accidents that are not adequately described by a ‘power
function’. To investigate what model forms may be appropriate the Hauer and Bamfo (1997)
integrate-differentiate (ID) method was followed. This lead to the trial of a combination of
exponential and power functions (as in the pedestrian model) and Hoerl’s function (as in the
rear-end model). An interesting property of these models is that they do not indicate that for
zero motor vehicle flows that the number of accidents is zero (although it does indicate that zero
pedestrian flows results in zero pedestrian accidents); this is a result of the more flexible model
form.
The reason why these exponential and combination model types fit better than power functions
for these accident types is that at low volumes there are very few accidents. This persists until
the flow increases to a point where far more accidents begin to occur. A power function is not
suitable for representing this type of relationship. Figure 7 illustrates this through a graph of
Hauer and Bamfo’s “Empirical integral function”; this represents the integral of the function
relating accidents to entering flow. The low increase in accidents until approximately 5000 vpd
means that a power function is not suitable. Even when using Hoerl’s function to predict rear-
end accidents, care must be exercised at very low flows due to the ‘zero accidents, zero flow’
problem.
All of the preferred models for motor-vehicle and pedestrians accidents include non-flow
variables. For the motor-vehicle entering versus cycle 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. This relationship implies
that the European approach to the design of roundabouts has safety benefits by reducing
vehicle speeds. 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%.
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 ‘other’ accidents.
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 6 along with the model for cyclist circulating accidents from
Turner et al. (2006b) and the model for accidents involving all wheeled road users (cyclists and
motor-vehicles) in Turner (2000).
A comparison between the preferred models in Table 5 and the flow-only models in Table 6
illustrate the effect of the correlation between circulating flow and mean circulating speed in the
models for motor-vehicle only accidents. This can be observed by the lower exponent for the
circulating flow in the preferred model when compared to the flow-only model.
Table 6 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 6: Entering-versus-circulating accident prediction models
Model
Study
Equation (accidents per approach)
Motor Vehicle Only
Accidents (Flow Only
Model)
Current
Study
37.048.0-5
11049.2 ceUMAR QQA
Motor Vehicle and
Cyclist Accidents
Turner 2000
41.042.0-4
11014.1 ceUWXR QQA
Cyclist Circulating
Accidents (Flow Only
Model)
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
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
66.04
1021.3
35.1
HS
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.
APPLICATIONS OF MODELS
There are a number of applications for accident prediction models, including the roundabout
accident prediction models presented above. The models can be used for performance
assessment of a current high and low speed roundabouts, by comparing the observed accident
rate with that predicted by the models, which is in effect a national or regional prediction of the
expected accident rate. Accident Prediction Models are used in New Zealand in economic
evaluation at new sites and at sites where there is a limited accident record, due to changes to
the site or low traffic volumes. These two applications will be covered in more detail below.
Accident prediction models are also used in New Zealand in the assessment of development
applications and for developing road safety strategies and policies. These applications are
discussed in the accompanying conference paper (Turner et al 2006a). Further applications of
the models are discussed in Turner et al. (2003).
Performance Assessment/Blackspot Identification
The majority of roading controlling authorities, including Transit NZ and Christchurch City
Council, have, or are in the process of developing, Safety Management Systems for their road
networks. These systems consist of a number of road safety processes that the road controlling
authority undertakes to identify, investigate, improve and monitor over their road network.
The majority of safety investigation programmes base the identification of accident trouble-
spots, and the monitoring of safety, on accident frequency. Given that higher volume sections of
the road network have a higher accident exposure, and therefore are more likely to have
accident clusters, most of the attention focuses on such sections. An enhancement to the
current investigation and monitoring systems would be to focus on accident risk and
consequences, which would also highlight high-risk sites, irrespective of the traffic volumes.
It is possible to incorporate accident risk and consequences within safety investigation
programmes using the following process:
Stage 1 Calculate the expected number of accidents (estimator of the UTAR) at each
intersection, on each link and at each ‘other’ site (e.g. railway crossing or bridge) within the
study network using accident prediction models.
Stage 2 Compare the observed number of accidents at each intersection, along each link and
at each ‘other’ site with the predicted accident rate.
Stage 3 Identify and investigate those sites (as part of accident reduction studies and a safety
audit program) that have observed accident rates that are significantly higher than the accident
predictions. Make improvements to the sites and monitor them by comparing future accident
occurrence with model predictions.
Stage 4 Identify and investigate those sites that have a high accident risk and/or high accident
consequences, irrespective of the traffic volumes. As funding allows, reduce the accident risk
and consequences at such sites through low cost accident remedial measures.
As engineers become more successful in addressing safety deficiencies at the black-spot and
black-route sites identified in Stage 3, the number of accident clusters will reduce and accidents
will become more spread out within road networks. This will lead to a greater focus on Stage 4
of the above process and on making small incremental steps towards reducing accident risk
over the entire road network.
The availability of accident prediction models enables road controlling authorities (RCAs) not
only to identify existing or potential accident trouble spots, but also to monitor accident risk over
their networks and compare the level of risk in their networks with other RCAs. The use of
APMs will improve the allocation of road safety improvement funding to areas with the most
need in the road network.
Economic Evaluation
In New Zealand all substantial road safety improvement projects submitted for central
Government funding (through Land Transport NZ) must be accompanied by an economic
evaluation. There are three economic evaluation methods available in New Zealand for
assessing the safety benefits of road safety treatments. Two of the methods, Accident Rate
Analysis and Weighted Accident Procedure (WAP) utilise Accident Prediction Models. This
paper discusses the latter WAP method.
The WAP makes use of the following equation:
Aw = w * AT + (1-w) * AS
where w = k/(AT + k) for generalised linear models with a negative binomial distribution. As is
the site-specific accident rate, which is the annual average number of historical accidents in the
last five years. AT is the typical or national accident rate predicted using an accident prediction
model and ‘k’ is a parameter of the negative binomial distribution.
The better the accident prediction model, reflected in a high risk k, relative to AT, the more
weight is placed on the typical accident rate. Low k-values occur for site types where there is a
high variability in accident observations. In this case more weight is placed on the historical
accident data.
The weighted accident rate procedure makes use of data from both sources and while taking
into account the expected accident risk at a site, also acknowledges that there are features of
sites that are unique and need to be considered when estimating the UTAR of the existing sites.
The UTAR of the upgraded site is calculated using one of two methods. If the change to the site
is fundamental (the pattern or severity of subsequent accidents at the site are expected to be
significantly different), then accident rate analysis is used for the economic evaluation, as the
accident history is no longer valid. In accident rate analysis a prediction model is used to
calculate the option accident rate (per year). In cases where the changes at a site are not
fundamental (similar types of accidents are expected) then a weighted procedure is used for the
option accident evaluation. The formula used for calculating the option accident rate is:
Aw (option) = AT (option) * (Aw (Do minimum)/AT (Do minimum))
More detail on this application can be found in Turner et al. (2005).
Changes in Form of Control
The accident prediction models can be used to evaluate the safety benefits (or disbenefits) of
converting an urban priority intersection to roundabout or traffic signal control. Furthermore,
models can be used to compare the changes in the major accident types.
For the traffic flows at the four-arm intersection illustrated in Figure 8, the annual number of
accidents of each type can be calculated for each form of control. This analysis used models
for motor-vehicle accidents at traffic signals and priority control from Turner (2000) and models
for pedestrian and cyclist accidents at traffic signals from Turner et al. (2006b).
The analysis shows that the predicted annual number of accidents for priority control is 1.51
acc/year, for signalised control 1.24 acc/year and for roundabout control 0.71 acc/year. Along
with the large changes in the total number of reported accidents there are also large changes in
the accident types and road user involvement. Table 7 illustrates the predicted numbers of
accidents by accident type and road user for the three control types. This shows that although a
roundabout is the safer form of control overall, with considerable safety benefits for motor-
vehicle users, it is likely to have a much higher number of injury accidents involving cyclists
compared to traffic signals.
Table 7: Predicted accidents for different forms of control
Intersection Type
Road Users
Accident Type
Predicted Annual
Accidents
Roundabout
0.71
Motor-vehicle only
Entering vs
Circulating
0.11
Rear-end
0.07
Loss-of-control
0.11
Other
0.04
Cyclist
Entering vs
Circulating
0.23
Other
0.06
Pedestrian
All Pedestrian
Accidents
0.08
Traffic Signals
1.24
Motor-vehicle only
Crossing (No turns)
0.29
Right turn Against
0.44
Rear end
0.09
Loss of control
0.05
Other
0.11
Cyclist
Same Direction
0.06
Right Turn Against
0.01
Other
0.05
Pedestrian
Intersecting
0.06
Right Turning
0.06
Other
0.02
Priority Crossroads
1.51
Motor-vehicle and
cyclist
Crossing (No turns)
0.89
Right turn Against
0.19
Crossing (Vehicle
turning)
0.13
Loss of control
0.06
All road users
Others
0.24
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. Multiplicate 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 model forms specified by Hauer and Bamfo (1997) have been used in addition to the
standard models used by the study team. The Hauer and Bamfo model forms allow greater
flexibility in the nature of the relationship modelled. Like all accident prediction models these
models should only be used over the flow ranges of the underlying dataset. However, further
care must be taken with these model forms as they can only be applied over a particular flow
range, because of peculiarities in the underlying accident relationships.
The preferred ‘non-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. There are also benefits possible through reduction of
visibilities. Further work, however, is required to explain why the models indicate that ‘other
motor-vehicle accidents may increase with reduced visibilities.
There are a number of applications of accident prediction models. Two applications of
roundabout accident prediction models have been outlined. The models can be used for
performance assessment sites and identification of high risk (but not necessarily high volume)
sites for crash reduction studies. This is an alternative to the picking of sites based on accident
frequency (Blackspot Identification). The models can also be used in economic evaluation. In
New Zealand the Weighted Accident (analysis) Procedure (WAP) makes use of the accident
history and accident prediction models. This is expected to lead to a better assessment of
accident benefits by acknowledging the stochastic nature of the historical accident rate at lower
flow sites.
REFERENCES
E. Hauer and J. Bamfo. “Two tools for finding what function links the dependent variable to the
explanatory variables” Published in Proceedings of ICTCT 97 Conference, November 5-7 1997,
Lund, Sweden (1997).
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. (2001).
Transfund “Ins and Outs of Roundabouts: Safety Auditor’s Perspective”. Transfund New
Zealand, Wellington, New Zealand. (2000)
S. A. Turner, “Estimating Accidents in a Road Network”, PhD Thesis, Dept of Civil Engineering,
University of Canterbury. (1995)
S. A. Turner. “Accident Prediction Models”. Transfund New Zealand Research Report No. 192,
Transfund NZ, Wellington, New Zealand. (2000)
S. A. Turner, P. D. Durdin, I.Bone and M.Jackett, “New Zealand Accident Prediction Models and
Their Applications”. 21st ARRB /REAAA conference. (2003)
S. A. Turner, I. Melsom and P. D. Durdin, “Land Transport NZ’s Proactive Accident Analysis
Method: Weighted Accident Procedure” Unpublished Paper presented to 2005 ITE conference,
Melbourne, Australia (available form the author Shane.Turner@beca.com) (2005).
S. A. Turner, and A. Roozenburg, “Roundabout Crash Prediction Models”, Draft Land Transport
NZ Research Report, Land Transport NZ, Wellington NZ, (2006)
S. A. Turner, A. Roozenburg, T. Francis and G. Wood, “Prediction Models for Pedestrians and
Cyclists Accidents”, Towards Sustainable Land Transport Conference, Wellington, NZ. (2004).
S. A. Turner, A. Roozenburg, and T. Francis, “Predicting Accident Rates for Cyclists and
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.
AUTHOR BIOGRAPHIES
Shane Turner
Shane is a Senior Associate in the consulting firm Beca Infrastructure Ltd. He is based in the
Christchurch office where he leads a team of seven transport professionals. He is also the
national transport research manager for Beca. Shane completed his BE (Hons) from the
University of Auckland in 1990 and his PhD, specialising in the development of accident
prediction models, at the University of Canterbury in 1995. He is a guest lecturer in the Master
of Transport course at the University of Canterbury. Shane has extensive experience, through
leading many road safety research projects, in safety related research, particularly the
development of accident prediction models.
Graham Wood
Graham’s current role is Professor of Statistics and Head of Department in the Department of
Statistics, Macquarie University, Sydney. Graham has worked at the University of Canterbury,
Central Queensland University and Massey University, prior to moving to Sydney. He is the
author of 80 internationally refereed papers in mathematics and statistics, about 40 other
miscellaneous publications and has co-authored two books. Graham has been involved with the
fitting and development of accident prediction models in New Zealand for the past fifteen years,
leading recently to the publication of three papers in the area. Graham’s current research
interests are in mathematical and statistical modeling, particularly in optimisation, traffic accident
prediction modeling and bioinformatics.
Aaron Roozenburg
Aaron Roozenburg is a transportation engineer in the Christchurch office of Beca Infrastructure
Ltd. After graduating with a Masters in Transportation Engineering from the University of
Canterbury in 2004, Aaron has worked as a consultant in the road safety, traffic engineering,
sustainable transport and research fields. Aaron has been involved in a number of accident
prediction modeling research projects including projects developing models for pedestrian and
cyclists at intersections and right-turn-against accidents at traffic signals. He is currently
involved in three studies; the accident benefits of installing cycle facilities, the safety impact of
visibility and speed on accidents at roundabouts and the development of a comprehensive
accident prediction model for rural link crashes.
FIGURES
North
6
5
4
3
1
2
Approach 1
Approach 2
Approach 3
North
6
5
4
3
1
2
Approach 1
Approach 2
Approach 3
Approach 4
8
9
12
11
10
7
1
3
2
4
5
6
7
8
20
19
18
1
7
10
9
11
12
15
14
13
16
North
Approach 1
Approach 2
Approach 3
Approach 4
Approach 5
Figure 1: Numbering convention for movements and approaches
Figure 2: Entering and circulating vehicle speeds
Entering
Vehicle
Speed
Circulating
Vehicle
Speed
Figure 3: Measurement of V
10
Multiple
Motor-
vehicle
49%
Single
Motor-
vehicle
18%
Pedestrian
8%
Cyclist
25%
Figure 4: Road user involvement in injury accidents at urban roundabouts
Entering
versus
Circulating
51%
Rear End
11%
Loss of
Control
19%
Cutting off
by moving
right
2%
Cutting off
by moving
left
4%
Pedestrian
8%
Others
5%
Figure 5: Injury accident types of reported crashes at urban roundabouts)
0
1
2
3
4
5
1 2 3 4 5
x
A
b = 1
b = 0.5
b = - 0.5
b = 1.5
b = 0
Figure 6: Relationship between accidents and predictor variable x for different values of
model exponents (b
1
)
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
05000 10000 15000 20000
AADT (Qe)
Cumulative 'Bin Area' (F(Qe))
Figure 7: Empirical integral function for rear-end accidents
Figure 8: Example daily motor-vehicle, cyclist and pedestrian flows
... The trend is rising from E1 (single entry and circulatory lane) to E3 (two entry and circulatory lanes). Positive association between entry width (number of lanes) and crash frequency was identified in several studies (9,19,37,44,45). Truck apron has a protective influence: its presence is associated with lower crash frequency or severity. ...
... Location in rural areas seems to increase crash severity, compared to urban roundabouts; the same holds for presence of pedestrian crossing. Both variables are likely to be associated with speed and vulnerability, as also reported by Turner et al. (44) or Sˇenk & Ambros (37). ...
... For example, Rodegerdts et al. (30) used a speed gun to measure freeflow speeds in specific points of through-pass trajectory. A similar approach was used also by Spacek (8), Turner et al. (44), and Kim & Choi (47). In addition to speed guns, some measured speeds of isolated vehicles also by video camera (48,49). ...
Article
Roundabouts are considered the safest intersection design; however, the safety effect may not be satisfactory at each specific roundabout. This is true especially in countries where roundabout design is a relatively new concept, such as in the Czech Republic. Specifically, most Czech roundabout crashes were found to occur on entries. This motivated the presented study to investigate how entry design parameters influence safety on Czech roundabouts and, if possible, use the findings to update current Czech roundabout design guidelines. To this end, the study comprised three analyses: crash-based safety performance functions, speed analysis, and finally safety performance functions which incorporated speed. All three analyses proved that entry design parameters have a statistically significant influence on safety, in terms of crash frequency, severity and speeds. Given the study objective, this fact should be considered in Czech roundabout design guidelines.
... The studies considered speed as a factor influencing the level of traffic safety on intersections with a circular roadway. Some studies also considered speed among explanatory variables when developing accident prediction models (safety performance functions, SPFs)for example (Turner et al., 2009;Chen et al., 2011;Chen et al., 2013) used average speed of vehicles in the roundabout roadway and found its statistically significant relationship to accident frequencies. ...
... Consistently with literature review (Turner et al., 2009;Chen et al., 2011;Chen et al., 2013;Ambros et al., 2016;Novák et al., 2018), the basic form of SPF was adopted, including exposure variable (AADT) and explanatory variables: presence of lane dividers (Yes/No) or speed profile data. For speed profile data different values. ...
... a) NZ (New Zealand) model(Turner et al., 2009) model(Chen et al., 2013) ...
Article
Roundabouts are one of the safest types of intersections. However, the needs to meet the requirements of operation, capacity, traffic organization and surrounding development lead to a variety of design solutions. One of such alternatives are turbo-roundabouts, which simplify drivers' decision making, limit lane changing in the roundabout, and induce low driving speed thanks to raised lane dividers. However, in spite of their generally positive reception, the safety impact of turbo-roundabouts has not been sufficiently studied. Given the low number of existing turbo-roundabouts and the statistical rarity of accident occurrence, the prevalent previously conducted studies applied only simple before-after designs or relied on traffic conflicts in micro-simulations. Nevertheless, the presence of raised lane dividers is acknowledged as an important feature of well performing and safe turbo-roundabouts. Following the previous Polish studies, the primary objective of the present study was assessment of influence of presence of lane dividers on road safety and developing a reliable and valid surrogate safety measure based on field data, which will circumvent the limitations of accident data or micro-simulations. The secondary objective was using the developed surrogate safety measure to assess and compare the safety levels of Polish turbo-roundabout samples with and without raised lane dividers. The surrogate safety measure was based on speed and lane behaviour. Speed was obtained from video observations and floating car data, which enabled the construction of representative speed profiles. Lane behaviour data was gathered from video observations. The collection of the data allowed for a relative validation of the method by comparing the safety performance of turbo-roundabouts with and without raised lane dividers. In the end, the surrogate measure was applied for evaluation of safety levels and enhancement of the existing safety performance functions, which combine traffic volumes, and speeds as a function of radii). The final models may help quantify the safety impact of different turbo-roundabout solutions.
... In total 196 roundabouts were used for Czech SPF (for details see [16]). The Czech SPF was compared to several other SPFs that were retrieved from international literature [9,[24][25][26][27][28]. They included European examples (Belgium, France, Italy, Sweden, United Kingdom) as well as United States, Canada and New Zealand -see Figure 4. ...
... Belgium [9] 1.10·10 -4 1.00 Canada [28] 3.05·10 -6 1.42 Czech Republic [16] 4.65·10 -2 0.43 France [28] 2.40·10 -7 1.40 Italy [26] 1.15·10 -8 1.86 New Zealand [27] 6.11·10 -4 0.58 United Kingdom [24] 8.00·10 -6 1.24 United States [25] 2.30·10 -3 0.75 Sweden [28] 3.08·10 -6 1.20 However, compared to other European countries (and New Zealand) Czech SPF performs worse in the whole range of AADT values. ...
Article
Full-text available
Roundabouts around the world are often seen as a beneficial measure for intersection safety. Although their number has grown recently in the Czech Republic, their safety impact has not been fully studied. Furthermore, Czech roundabouts have sometimes been unpopular, including doubts about their benefits. This situation inspired the authors to investigate three questions related to Czech roundabouts: (1) Are roundabouts safer than traditional intersections?; (2) Are roundabout conversions beneficial for safety?; (3) Is Czech roundabout safety performance comparable to other countries? Safety performance functions were developed based on data samples and used in order to answer these research questions. The final results are mixed: roundabouts seem to be safer compared to traditional intersections and before-after study of urban roundabout conversions yielded positive crash modification factors; on the other hand, expected crash frequencies on Czech roundabouts are higher compared to other European countries.
... Similarly, the geometry of roundabouts appears to play an important role in crash severity; geometry effects on approach, entry and impact speeds are implicated. The New Zealand Transport Agency (NZTA) investigated the statistical relationships between crashes, speed, traffic volume and sight distance at roundabouts (Turner, Roozenburg & Smith 2009). This study suggested some of the factors associated with casualty crashes at rural roundabouts: ...
Technical Report
Full-text available
Intersection crashes account for approximately 30% of severe injuries in Australia and New Zealand. This study sought to improve understanding of the key factors in intersection severe injury crashes, and to develop initiatives to improve the design of intersections for better alignment with the Safe System objective of minimising death and serious injury. The study reviewed recent literature and data to synthesise the following Safe System intersection design principles: minimise conflict points, remove/simplify road user decisions, minimise impact angles, and minimise entry and impact speeds. Using inputs from literature and data findings, a new safety analytical method, and practitioners, the study proposed nine innovative intersection design concepts seeking to increase Safe System alignment across a wide range of scenarios (urban/rural, new/retrofit). These design concepts form a starting point for practitioners' trials and refinement.
... The studies consider speed as a factor influencing the level of traffic safety on intersections with a circular roadway. This is indicated by the studies carried out with the use of accident prediction models where among independent variables the average speed of vehicles in the roundabout roadway is considered statistically significant [11], [12], [13]. ...
Conference Paper
Full-text available
Roundabouts are one of the safest types of intersections. However, the needs to meet the requirements of operation, capacity, traffic organization and surrounding development lead to a variety of design solutions. One of such alternatives are turbo-roundabouts, which simplify drivers' decision making, limit lane changing in the roundabout, and induce low driving speed thanks to raised lane dividers. However, in spite of their generally positive reception, the safety impact of turbo-roundabouts has not been sufficiently studied. Given the low number of existing turbo-roundabouts and the statistical rarity of accident occurrence, the prevalent previously conducted studies applied only simple before-after designs or relied on traffic conflicts in micro-simulations. Nevertheless, the presence of raised lane dividers is acknowledged as an important feature of well performing and safe turbo-roundabouts. Following the previous Polish studies, the primary objective of the present study is to develop a reliable and valid surrogate safety measure based on field data, which will circumvent the limitations of accident data or micro-simulations. The secondary objective will be to use the surrogate safety measure to assess and compare the safety levels of Polish turbo-roundabout samples with and without raised lane dividers. The surrogate safety measure is based on speed and lane behaviour. Speed was obtained from video observations and floating car data, which enable the construction of representative speed profiles. Lane behaviour data were gathered from video observations. The collection of the data allowed for a relative validation of the method by comparing the safety performance of turbo-roundabouts with and without raised lane dividers. In the end, the surrogate measure was applied for evaluation of safety levels and will also enhance the existing accident prediction models, combining traffic volumes, trajectory geometry and speeds. The final models can help quantify the safety impact of different turbo-roundabout solutions.
... In rural areas, the speed of the vehicle is normally high, so providing safe sight distance is crucial. The sight distance of the roundabout depends on the approach speed and negotiating speed (Turner et al. 2009). One study investigated driver sight distance as an independent variable to predict passenger vehicle speed and vehicle crash rates at 26 singlelane roundabouts. ...
Article
This research was focused on two issues related to multilane roundabouts on high-speed highways (speed limit 45 mph or greater) in rural and suburban areas. The first was the tradeoff between converting a traditional stop-controlled or signalized intersection to a multilane roundabout while the second was the safety of newly constructed high-speed multilane roundabouts in rural and suburban areas. The research team reviewed information from diverse published documents and conducted a survey of state and local transportation agencies. Crash data on multilane rural roundabouts were not available for this research. Therefore, the research team relied on crash and other data for single lane roundabouts that were constructed to replace rural two-way stop-controlled intersections in Kansas. To gain further insights into the safety of rural multilane roundabouts, the research team focused on investigating the safety of urban multilane roundabouts from published sources. Results of the survey indicated the need for proper design of roundabouts including signage and lighting and the potential for gaining benefits from public informational campaigns. Results of the Kansas data analysis of single lane roundabouts showed that overall all types of crashes were reduced after conversion of TWSC intersections to modern single lane roundabouts. Total crashes decreased by 58.13%; fatal crashes were reduced by 100% at all locations and non-fatal injury crashes were reduced with an overall reduction rate of 76.47%. Property-damage-only crashes were reduced by 35.49% as a whole, but two out of the four analyzed sites experienced increases in property-damage-only crashes after conversion to roundabouts. The annual value of the reduction in comprehensive crash costs from conversion of a two-way stop-controlled intersection on a rural, high-speed highway to a single lane modern roundabout was between $1.0 million and $1.6 million in 2014 dollars. A review of multilane roundabout conversions (mostly in urban areas) showed safety improvements compared to signalized and two-way stop-controlled intersections. Recommendations are presented in the report.
... Brüde and Larsson (1999a) find that the accident rate for cyclists is twice as high at small roundabouts, where the central island including truck apron is less than 20 m, compared to larger roundabouts. Hels and Møller (2007) and Turner et al. (2009) find that as motorists' entry and circulation speed at roundabouts increase then bicycle safety worsens. ...
Article
May roundabouts be safer for cyclists than intersections? How are safe roundabouts designed? This paper tries to answer these questions on the basis of a before-after safety study of conversions of intersections to 255 single-lane roundabouts in Denmark. The before-after study accounts for long-term accident and injury trends and regression-to-the-mean effects. In order to relate safety effects for cyclists of various roundabout design features it is crucial to split the converted sites by speed limit, because safety effects for both cyclists and other road users of converting intersections to roundabouts depend heavily on speed limits on roads entering the converted sites. If speed limits are 70 km/h or higher then converting intersections to roundabouts have resulted in bicycle safety improvements in Denmark. Results show that diameter and height of central islands and type of bicycle facilities at single-lane roundabouts have considerable impacts on cyclists’ safety. Central island diameters of 20–40 m are safer for cyclists than smaller or larger roundabouts. A central island, which middle is elevated 2 m or more above the circulating lane, is safer for cyclists than single-lane roundabouts with lower central islands. Single-lane roundabouts with separate cycle paths, where cyclists must yield to motorists entering or exiting the roundabout, are safer than roundabouts with cycle lanes. Single-lane roundabouts are safer for cyclists than intersections regardless of speed limits when these roundabouts have high central islands and/or separate cycle paths.
Article
Full-text available
School is one of the most important institutions of socialization and is very reliable in refining the young people in accordance with the philosophy of society and provides them with the most important skills that allow them to interact positively with the environment in which they coexist. Hence, one can state that the school is the best medium to pass any message of public benefit. In this study, we examined the effectiveness of educational campaign in improving the behavior of the pedestrians. They are a sample of 86 pupils from two secondary schools (Al-Naeem Al-Naimi and Tahiri Abderrahman) in Djelfa city. These two secondary schools were selected intentionally, because they are on the road and close to a crossroad equipped with traffic signals and police. In this study, the researcher adopted a semi-experimental design using network observation as a tool for data collection. The results of T-test were statistically significant at 0.01 and supported the hypotheses of the study where it was confirmed the effectiveness of the educational campaign and the existence of differences between females and males in response to the campaign in favor of females.
Conference Paper
Full-text available
The management of speed is considered an important safety issue at roundabouts. The approach speed and negotiating speed through roundabouts depends on both the geometric design of the roundabout and sight distance. In New Zealand and in the Australian States the design standards (based on Austroads) recommend long approach sight distance and provision of relatively high design speeds. This is in contrast to European roundabouts where visibility is normally restricted and the geometric design encourages slow approach and negotiation speeds. The 'flow-only' models developed in Turner (2000) have been extended to include sight distance, intersection layout and observed speed variables. Models have been produced for the major motor-vehicles only, pedestrian versus motor-vehicles and cyclists versus motor-vehicle accident types. 'Flow-only' models have also been produced for roundabouts on roads with high speed limits. The models have then been used to assess whether a move towards the European design philosophy, of lower speed and reduced sight distance, is likely to produce lower accident rates for all modes, particularly in areas that have high concentrations of pedestrians and cyclists.
Chapter
Full-text available
The popularity of roundabout application around the world is evident. Due to the inexperience of construction companies and the lack of proper national guidelines, distinctiveness in design is noticeable. In some intersections this led to reduction of Traffic (operational) Efficiency (TE). The purpose of this paper is to analyze: 1) the current state of roundabouts in Croatia; (2) known approaches to using geometry elements of roundabouts to predict TE; (3) overview and comparison of selected design guidelines; and (4) to present and comment the latest Croatian Roundabout Design Guidelines on State Roads 2014 and show examples of good practice. Research results will serve to disseminate the knowledge for proper application and implementation of national roundabouts in order to compare it with international design practice and standards.
Conference Paper
Full-text available
During the late 1990s generalised linear models were developed for the major motor-vehicle-only accidents types at urban intersections in New Zealand. Generally there were four or five major motor-vehicle-only accident types. At traffic signals and roundabouts, particularly in Christchurch, the next most common accident types often involved pedestrians or cyclists. This paper discusses a study undertaken for Transfund to produce generalised linear models for the major cycle and pedestrian accident types. A sample of traffic signal, roundabout and mid-block locations was selected in Christchurch, Hamilton and Palmerston North; three flat cities within New Zealand with a relatively high number of cyclists. The mid-block selection criteria focused on arterial routes with 'strip' shopping. Pedestrian and cycle count data was collected at each site, to accompany motor-vehicle count data. Data was collected on a number of non-flow variables, including visibility, number of approach lanes, crossing distance, and compliance with signalised crossing 'green man'. Using the collected and existing data, accident prediction models were developed using generalised linear models for accidents between cyclists and motor vehicles and pedestrians and motor vehicles. These models are presented in this paper.
Technical Report
Full-text available
The management of speed is considered an important safety issue at roundabouts. The approach speed and negotiating speed through roundabouts depends on the geometric design of the roundabout and sight distance. In New Zealand and in Australia, the design standards recommend long approach sight distances and provision of relatively high design speeds. This is in contrast to European roundabouts, where visibility is normally restricted and the geometric design encourages slow approach and negotiation speeds. This work, undertaken in 2006, extends previous research by the authors developing crash prediction models at roundabouts to include sight distance, intersection layout and observed speed variables. Models have been produced for the major motor vehicles only, pedestrian versus motor vehicles and cyclists versus motor vehicle crash types. Flow-only models have also been produced for roundabouts on roads with high speed limits. The models produced indicate that roundabouts with lower speeds (observed and speed limit), fewer approach lanes and reduced visibility have lower crash rates.
Conference Paper
Full-text available
This paper reviews model relating accidents to traffic flows, with particular emphasis on the appropriateness of the model form and the statistical technique employed for parameter estimation. The development of generalised linear models for predicting accidents at intersections in New Zealand is then described. It is shown that the new models fit the empirical data better than existing, simpler models. the use of the models for predicting intersection accidents in three networks is described.
Conference Paper
Full-text available
Pedestrians are over-represented in the New Zealand accident statistics given the proportion of walking trips, particularly on a distance-travelled basis. In this study Beca developed prediction models for pedestrian versus motor vehicle accidents at urban traffic signals and mid-block road sections in commercial (shopping) areas. The accident rate for pedestrian accidents was significantly higher than for motor vehicle only accidents. However, the research indicates that the pedestrian accident risk (accidents per pedestrian) drops considerably when the pedestrian volume increases, and this drop is much more pronounced than for motor vehicle only accidents. During the study information on pedestrian only and off-road pedestrian accidents was also collected from hospital interviews and other data sources. This paper will present the accident prediction models produced for each accident type, pedestrian hourly profiles and a number of other tables and graphs that illustrate trends in accident occurrences.
Conference Paper
Full-text available
The management of speed is considered an important safety issue at roundabouts. The approach speed and negotiating speed through roundabouts depends on both the geometric design of the roundabout and sight distance. In New Zealand and in the Australian States the design standards (based on Austroads) recommend long approach sight distance and provision of relatively high design speeds. This is in contrast to European roundabouts where visibility is normally restricted and the geometric design encourages slow approach and negotiation speeds. The 'flow-only' models developed in Turner (2000) have been extended to include sight distance, intersection layout and observed speed variables. Models have been produced for the major motor-vehicles only, pedestrian versus motor-vehicles and cyclists versus motor-vehicle accident types. 'Flow-only' models have also been produced for roundabouts on roads with high speed limits. The models have then been used to assess whether a move towards the European design philosophy, of lower speed and reduced sight distance, is likely to produce lower accident rates for all modes, particularly in areas that have high concentrations of pedestrians and cyclists.
Conference Paper
Full-text available
Accident prediction models have been developed for urban and rural intersections and links and for other sites, including isolated curves, narrow bridges, single lane bridges and railway crossings. This paper details the generalised linear models that have been developed from reported injury accident data and traffic counts in New Zealand for major accident types and total accidents. Refinements have been made to the methods used to calculate the goodness-of-fit testing statistic, the deviance function to address the low mean value problem. Applications of the models are discussed, including economic evaluation, performance measures and safety management systems, and optimisation networks flow patterns to improve safety.
Article
The report gives the findings of a study of personal injury accidents at a sample of 84 four-arm roundabouts on main roads in the UK. The study includes small roundabouts and roundabouts of conventional design, in both 30-40 and 50-70 mile/h speed limit zones. Tabulations are given showing accident frequencies, severities, and rates by roundabout type. The accidents are further analyzed by type (entering-circulating, approaching, single-vehicle, etc) and by road-user involvement (cyclist, motorcyclist, pedestrian, etc). The accident frequencies by type are related to traffic flow and roundabout geometry using regression methods. Equations are developed to enable roundabout accidents to be predicted for use in design and appraisal.
Article
Traffic accidents prediction has an important meaning to the improvement of traffic safety management, and urban traffic accidents prediction model. Different approaches for developing Accident Prediction Models (APMs) are used such as multiple linear regression, multiple logistic regression, Poisson models, negative binomial models, random effects models and various soft computing techniques such as fuzzy logic, artificial neural networks and more recently the neuro-fuzzy systems. This paper reviews application of these approaches for developing APMs and advantages of neuro-fuzzy system in modelling accidents in urban road links and intersections.