Crash patterns and potential engineering countermeasures at Maryland roundabouts

February 2009

Objectives: Each year in the United States more than 1.2 million injuries and 9,000 deaths occur in motor vehicle crashes at intersections. Previous research has found that construction of roundabouts in place of traditional intersections can decrease crash frequency and severity. Despite these safety benefits, some crashes still occur at roundabouts. The present study systematically reviewed police crash reports for a set of roundabouts in Maryland to develop a typology of crashes and identify potential countermeasures. Methods: A total of 283 crash reports were reviewed including 149 crashes at 29 single-lane roundabouts and 134 crashes at 9 double-lane roundabouts. Based on the police reports, crash types were developed and examined by type of roundabout (single-lane, double-lane), crash location within the roundabout (entrance, circular roadway, exit), and other variables. Field observations were conducted at 8 roundabouts with above-average crash histories to aid in identifying potential countermeasures. Results: About three quarters of the crashes involved only property damage. Of the injury crashes, 14% involved at least one disabling injury; the remaining crashes resulted in probable injuries (36%) or nonincapacitating injuries (49%). One common crash pattern at both single- and double-lane roundabouts involved vehicles colliding with the central island, which accounted for almost half of all single-vehicle run-off-road crashes. Other major crash types included rear-end and sideswipe collisions. About three quarters of all collisions occurred at entrances to roundabouts. Based on review of crash reports and visits to several roundabouts, high approach speeds were an important driver crash factor, and some drivers may not have seen the roundabout in time. Conclusions: Increasing the conspicuity of upcoming roundabouts through larger "roundabout ahead" and "yield" signs could reduce speeds by alerting drivers ahead of time, especially at night. Enhanced landscaping of central islands as well as reflective pavement markers and yield signs at the entrance to roundabouts also could help drivers recognize roundabouts and the need to yield to circulating traffic. Certain design features (e.g., entry deflection on approach roads) also may aid in reducing speeds.