The effect of roundabout design features on cyclist accident rate

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Abstract

Roundabouts are known to result in fewer traffic accidents than traditional intersections. However, this is to a lesser degree true for bicycles than for vehicles. In this paper, we aimed at establishing statistical relationships through Poisson regression and logistic regression analyses between yearly rate of cyclist accidents on one hand and roundabout geometry, age and traffic volume (vehicles and cyclists) on the other. We related all roundabout cyclist accidents recorded by the hospital emergency department of the town of Odense, Denmark, through the years 1999–2003 (N = 171) to various geometric features, age and traffic volume of all roundabouts on the Danish island of Funen (N = 88). Cyclist and vehicle volumes turned out to be significant predictors in most of our models—the higher the volumes, the more accidents. Moreover, potential vehicle speed was a significant predictor, and so was age of the roundabout—older roundabouts related to more accidents and higher accident probability. Excluding 48 single cyclist accidents strengthened the relationship between accidents on one hand and vehicle and cyclist volume and potential vehicle speed on the other. This stresses the significance of speed and traffic volume for traffic accidents with more than one partner involved. The 48 single cyclist accidents were significantly related to the traffic volume of cyclists only. Due to our limited number of observations, the models should be regarded as indicative.

Introduction

Roundabouts are popular. During the latest 20 years, more and more intersections have been converted to roundabouts, both in Europe (e.g. Britain: Brown, 1995, Denmark: Jørgensen and Jørgensen, 1994, Finland: Räsänen and Summala, 2000, Sweden: Brüde and Larsson, 2000, Switzerland: Huber, 1995, The Netherlands: Schoon and van Minnen, 1994, France: Guichet pers. com.) and in the US, although the development in the US has come later than in Europe (Perseaud et al., 2001, Flannery, 2001). The advantages of roundabouts as compared to intersections concern two aspects: safety and mobility. Safety: roundabouts produce slowed and smooth traffic flows with few stops and no left turns in front of left coming traffic in countries where traffic drives on the right. Mobility: roundabouts have a larger capacity than give way intersections and signalised junctions due to the fact that left turns are omitted (Elvik and Vaa, 2004). Elvik and Vaa (2004) have reviewed 34 studies from Northern Europe, Australia and the US evaluating the effect on accidents of converting an intersection into a roundabout. They find a mean reduction in injury accidents of 10 to 40%. Generally, the reduction in serious injury accidents is larger than that in less serious injury accidents, while property damage only accidents typically increase in number, although this effect is highly uncertain. Roundabouts, however, do not have the same positive effect on the number of cyclist accidents (Schoon and van Minnen, 1994, Jørgensen and Jørgensen, 1994). There may be more than one explanation to this. Firstly, a roundabout represents a more challenging orientation task for all road users than an ordinary junction because of the circular design. This may increase the probability of orientation failure and the risk of accidents. Secondly, passing through a roundabout is physically more demanding due to the circular deflection of the road. This may increase the number and proportion of one-cyclist accidents.

The aim of this study was to identify reasons for the relatively higher prevalence of cyclist accidents in Danish roundabouts compared to intersections (Herslund and Jørgensen, 2003). Our primary aim was to find relations between the number of cyclist accidents in roundabouts and the geometric features of a roundabout, its age and traffic volume (vehicles as well as cyclists). Secondly, we wanted to clarify the types and prevalence of various cyclist accidents. Thirdly, our aim was to quantify the degree of underreporting of cyclist accidents (cyclist accidents recorded by the police compared to the cyclist accidents recorded by the hospital emergency department).

The main hypotheses of this study were:

  • The number of accidents with cyclists in a roundabout increases with increasing of the traffic volume of cyclists and vehicles.

  • Roundabout geometry, which allows higher speed of the vehicles leads to higher number of cyclist accidents.

  • Existence of a cyclist facility makes the roundabout safer for cyclists if the traffic volume of vehicles is high.

Section snippets

Data

The data set consisted of data on cyclist accidents and data on geometric features, age and position of the roundabouts, besides the daily number of vehicles and cyclists entering the roundabouts (measured per 24 h).

The cyclist roundabout accidents were recorded in two different ways: by the police and by the emergency department of the hospital in Odense, Funen, in Denmark. Accidents involving cyclists between the years 1999 and 2003 on the Danish island of Funen were used.

For all roundabouts

Data analysis

The goal of the analysis was to explain as much variation as possible in the number of cyclist accidents in the roundabouts by the independent variables mentioned above. We have used a Poisson model with the formal expression:log(y)=β0+β1x1+β2x2++βnxn+ε,where y is the dependent variable (number of accidents per year after the construction of the roundabout), x1, …, xn denote the independent variables mentioned above, and β0, …, βn are coefficients scaling the variables. Binary variables (e.g.

Results

All roundabouts at the island of Funen were included in the analysis. Details on the variables describing them are summarised in Table 1, Table 2.

A total of 171 accidents involving cyclists were recorded by the emergency department of the hospital of Odense through the period 1999–2003, 152 of which were unambiguously assigned to a given roundabout. Only 42 of these 171 cyclist accidents were recorded by the police. Of the 171 accidents recorded by the emergency department, 69 (40%) were

Discussion

The 88 roundabouts included in this analysis differ considerably, both in terms of geometry and traffic volumes. All of them are built in the speed-reducing way with legs more or less perpendicularly radiating from the circulatory roadway as opposed to the dynamic design that gives the roundabout a higher capacity but is not recommended where there are many cyclists (Jørgensen and Jørgensen, 2002).

Single-cyclist accidents were the type of accident most seriously underreported by the police,

Acknowledgements

We thank Lars Binderup-Larsen, consultant orthopaedic surgeon, for a great help in extracting relevant data from the databases of the Accident Analysis Group of the Odense University Hospital. Moreover, we thank all employees of the road authorities who provided us with invaluable information: The Danish Road Directorate, The County and the Communities of Funen. This work was supported by a grant from the Danish Ministry of Transport and Energy.

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