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Report numberRA-MOW-2008-001
TitleInjury accidents with bicyclists at roundabouts
SubtitleInfluence of the design of cycle facilities and other location characteristics
AuthorsStijn Daniels
Tom Brijs
Erik Nuyts
Geert Wets
Published byPolicy Research Centre for Mobility and Public Works, track Traffic Safety 2007-2011
Number of pages27
Date31/03/2008
ISBN
Document languageEnglish
Partner(s)Universiteit Hasselt
Work packageOther: Infrastructure and space
Summary

Roundabouts in general have a favourable effect on traffic safety, at least for crashes causing injuries. A meta-analysis on 28 studies in 8 different countries revealed a best estimate of a reduction of injury crashes of 30-50%. Other studies delivered similar results. All those studies reported a considerably stronger decrease in the number of severest crashes (fatalities and crashes involving serious injuries) compared to the decrease of the total number of injury crashes.

 

Less is known about the safety effects of roundabouts for particular types of road users, such as bicyclists. Roundabouts seem to induce a higher number of bicyclist-involved crashes than might be expected from the presence of bicycles in overall traffic. In Flanders-Belgium bicyclists appear to be involved in almost one third of reported injury crashes at roundabouts while generally only 14.6% of all trips (5.7% of distances) are made by bicycle. The apparent overrepresentation of bicyclists in crashes at roundabouts was the main cause to conduct an evaluation study on the effects of roundabouts, more specifically on crashes involving bicyclists.

 

Some basic design types of cycle facilities at roundabouts can be distinguished. They are ordered into four categories: mixed traffic, cycle lanes, separate cycle paths and gradeseparated cycle paths.

 

A sample of 90 roundabouts that were constructed between 1994 and 2000 in the Flanders region of Belgium was studied. Both single-lane as well as double-lane roundabouts occur in the sample, although single-lanes are far more common. 21 of the 90 roundabouts were replacing traffic signals. The other roundabouts were built on other types of intersections (intersections with stop signs, give way-signs or general priority to the right). Furthermore the colour of the cyclist facility (when present) was noticed. In Flanders it is common to colour cyclist facilities red, although it is not compulsory.

 

Two comparison groups were composed, consisting of 76 intersections inside built-up areas and 96 intersections outside built-up area respectively serving as a comparison group for roundabouts inside and outside built-up areas. Detailed crash data were available from the National Statistical Institution for the period 1991-2001. Only crashes where at least one bicyclist was involved were included. Crashes were divided into 3 classes based on the severest injury that was reported: crashes involving at least one fatally injured person (killed immediately or within 30 days after the crash), crashes involving at least one seriously injured (person hospitalized for at least 24 hours) and crashes involving at least one slightly injured.

 

The adopted methodology was that of an Empirical Bayes - before and after study. The use of comparison groups allowed to control for general trends in traffic safety and possible regression-to-the-mean effects. The before-and after design allowed to determine effectiveness-indices for each roundabout in the sample. Since additional data about geometric features of the roundabout were available some regression models could be fitted in order to explain the variance of the estimated values of the effectivenessindices according to changes in factors such as number of lanes, pavement colour, location inside/outside built-up area etc.

 

The best estimate for the overall effect of roundabouts on injury crashes involving bicyclists on or nearby the roundabout is an increase of 27%. The best estimate for the effect on crashes involving fatal and serious injuries is an increase of 42-44%.

 

The number of injury crashes at roundabouts with cycle lanes turns out to increase significantly (+93%, C.I. [+38%;+169%]. However, for the other 3 design types (mixed traffic, separate cycle paths, grade-separated cycle paths) the best estimate is a decrease in the number of crashes (-17%), although not significant. However, regarding the severest accidents, the aggregated results for each of the design types show an increase in the number of fatal and serious crashes.

 

Linear regression models were fitted in order to estimate the relationship between the estimated value for the effectiveness per location and some known characteristics of the roundabout locations. It is concluded that the presence of a cycle lane or the presence of traffic signals in the before-situation do increase the likelihood of a deterioration after a roundabout is constructed.

 

In the study data, the presence of cycle lanes correlates with a higher value of the effectiveness-index reflecting an estimated increase in the number of crashes. This effect was earlier suggested in a German study. A Dutch before and after-study found no major differences in the evolution of crashes with bicyclists between three different roundabout design types (mixed traffic, cycle lanes, separate cycle paths). Regarding to numbers of victims however, it was concluded that at roundabouts with a considerable traffic volume, a separate cycle path design was safer than both other types. Therefore a separate cycle path design was recommended. In a recent Danish study no significant effect was found of the presence of a cycle facility (without distinction of different types) on the number of bicyclist crashes.

 

Regarding the severest crashes, the ones with fatally or seriously injured, the results that are presented in this paper deviate from existing knowledge. The results show an overall significant and substantial (best estimate around 42%) increase in the number of severe bicyclist crashes.

 

The main conclusions of this study can be summarized in four points:

  1. The data for the study sample suggest that the construction of a roundabout raises in general the number of severe injury crashes with bicyclists, regardless of the design type of cycle facilities.
  2. Regarding the effects on all injury accidents, roundabouts with cycle lanes perform worse compared to the three other design types (mixed traffic, separate cycle paths and grade-separated cycle paths).
  3. Roundabouts that are replacing signal-controlled intersections seem to have had a worse evolution compared with roundabouts on other types of intersections.
  4. Further research is needed in order to assess the validity of the results in different settings, such as other countries and other traffic conditions (e.g. depending on the prevalence of bicyclists in traffic). Further research is also needed in order to reveal possible causal mechanisms for crashes with bicyclists at roundabouts.

No decisive answer can be given about which recommendations should be given to road authorities, based on the present knowledge of safety effects of roundabouts. The value of roundabouts as an effective measure to reduce injury crashes for the full range of road users has been well proven. However, the contrast with the effects on the subgroup of crashes with bicyclists is remarkable and may cause a dilemma in policy making. Based on the results for the severest crashes, it would not be recommendable to construct a roundabout anyway when safety for bicyclists is a major concern. However, based on the results for all injury crashes, a clear distinction should be made between roundabouts with cycle lanes and other types of cycle facilities.

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