"Roundabouts: An Informational Guide" (FHWA-RD-00-067) was published in June 2000. To reach a wider audience, this presentation highlighted information, findings and recommendations in the Guide pertaining to the safety performance of roundabouts, including geometric design to promote speed consistency amongst various users. The AASHTO relationship between radius, super-elevation, side friction and speed can be used to estimate the consecutive speeds of a single vehicle through a roundabout design, as well as providing comparisons between speeds of conflicting users at the points of conflict. The Guide provides appropriate thresholds for the maximum absolute speeds, and recommended differences in these speeds. A planning-level comparison is provided on expected crash frequencies of roundabouts compared with urban/rural two-way stop-controlled and signalized intersections. This indicates that the relative safety potential of four-legged roundabouts are most likely to be realized at approach AADT's of less than 50,000 vpd, that is in the realm of roundabouts with single and double-lane approaches. Since the Guide was published, further before-after research on U.S. Roundabouts have confirmed their safety benefits through development and use of U.S.-based crash prediction models.
Abstract