Comparing safety at signalized intersections and roundabouts using simulated rear-end conflicts

The safety implications of adopting roundabouts in place of conventional signalized intersections have not been adequately assessed. A microscopic simulation model is used to compare the pattern of traffic conflicts at roundabouts with conflicts for signalized intersections. Three indicators of safety performance are defined: 1) “time to collision” (TTC), 2) “deceleration rate to avoid the crash” (DRAC), and 3) “crash potential index” (CPI). For each indicator, traffic conflict profiles are obtained in terms of: number of vehicles in conflict and number of conflicts per vehicle for selected directional manoeuvres. The exposure time to conflict for each manoeuvre and vehicle is also determined. A total of 12 combinations of geometric and traffic attributes (traffic scenarios) were simulated over a 15 minute period. The results suggest that roundabouts yield reduced exposure times to rear-end conflicts as compared to signalized intersections. On average, signalized intersections also reflected increased number of vehicles in conflict and percentage vehicles in conflict compared to roundabouts. This relationship was found to be independent of input volumes and pavement surface condition, and applied consistently to all safety indicator measures (TTC, DRAC and CPI).