Effects of Roundabouts on Traffic Safety for Bicyclists: Observational Study

Abstract

A before-and-after study of injury accidents with bicyclists on 95 roundabouts in Flanders-Belgium was carried out. The study design accounted for effects of general safety trends and regression-to-the-mean. Conversions of intersections into roundabouts turn out to have caused a significant increase of 29% (95% C.I. [2%; 63%]) in the number of injury accidents with bicyclists. The increase is even higher for accidents involving fatal or serious injuries (49-50%). Compared to the formerly proven favourable effects of roundabouts on safety in general, this result is unexpectedly poor. However, the effects of roundabouts on bicycle accidents differ depending on when these roundabouts are built inside or outside urban areas. Inside urban areas the construction of a roundabout did increase the number of injury accidents involving bicyclists by 48% (95% C.I. [10%; 100%]). For accidents inside urban area with fatal or serious injuries, we see an average increase of 80 to 81%. Outside urban areas however the zero-hypothesis of ‘no safety effect for bicyclists’ cannot be rejected (best estimate: + 5% accidents, not significant).