Safety Effects of Converting Intersections to Roundabouts

Abstract

A before–after crash study of converting intersections to roundabouts was carried out in Denmark. Correction factors were estimated on the basis of 32 general comparison groups in this nonexperimental observational study. The correction factors were used to account for general crash trends and regression-to-the-mean effects. The study included 332 converted sites, 57 fatalities, 1,271 other injuries, and 2,497 crashes. Conversions resulted in decreases in the numbers of crashes and injuries of 27% and 60%, respectively. Crashes became less severe—fatalities decreased by 87% and property damage–only crashes decreased by 16%. Safety effects varied between sites. The safety effects were better when the speed limit on the roundabout arms was higher. As the share of left-turn and angle crashes at intersections increased, the safety effects improved. As the share of bicycle crashes at intersections decreased, the safety effects improved. Central islands more than 6.6 ft (2 m) high produced better safety effects compared with lower central islands. Triangle or trumpet splitter islands produced better effects than no or parallel splitter islands. The numbers of bicycle crashes and injured cyclists increased by 65% and 40%, respectively, because of the conversions. Cycle lanes next to the circulating lane produced the worst safety effects for cyclists, whereas cycle paths without priority to cyclists resulted in the best effects. Colored cycle lanes and blue cycle crossings produced worse safety effects for cyclists than comparable bicycle facilities in lanes without color.