"Don't wait for accidents to happen"

October 25 – 26. 2007. Valencia

Safety effects of roundabouts: influence of the design of bicyclist facilities


During the last decades several studies were carried out into the effects of roundabouts on traffic safety. Roundabouts in general have a favourable effect on traffic safety, at least for accidents causing injuries. Especially the number of severe accidents (fatalities and accidents involving serious injuries) appears to decrease after converting intersections into roundabouts. The effects on property-damage only accidents are however highly uncertain.
Less is known about the safety effects of roundabouts for particular types of road users, such as bicyclists. In Flanders-Belgium, bicyclists appear to be involved in almost one third of reported injury accidents at roundabouts (1118 accidents with bicyclists; 3558 in total, period 1991-2001), while in general only 14.6% of all trips (5.7% of distances) are made by bicycle. The apparent overrepresentation of bicyclists in accidents on roundabouts was the main cause to conduct an evaluation study on the effects of roundabouts, specifically on accidents involving bicyclists.
A before and after study using a comparison group on a sample of 91 roundabouts was conducted in order to evaluate the effects on accidents with bicyclists. This study revealed a significant increase in the number of injury accidents with bicyclists, especially on locations inside built-up areas, after the construction of a roundabout. The increase was even higher when only accidents with severe injuries or fatalities were included. Moreover the design of the intersection in the before-situation seems to be important. Intersections that were signal-controlled in the before-situation appear to perform worse compared to other types of intersection design.
A next step was to look more into detail to the design type of cyclist facilities that are present at the investigated roundabouts. Basically 4 different types of cyclist facilities can be distinguished: the “mixed traffic” solution where motorised traffic and bicyclists are mixed together; adjoining – close to the roadway – cycle lanes; physically separated cycle paths (some meters away from the roadway); fully grade-separated crossings where conflicts from bicyclists with motorized traffic are virtually excluded. The results of this follow-up study will be presented during the workshop.

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