Effects of Phi and View Angle Geometric Principles on Safety of Multi-Lane Roundabouts

Abstract

Conventional intersections that have been replaced by multi-lane roundabouts have experienced a substantial reduction in both injuries and fatal crashes. Property-damage-only (PDO) crashes have also been reduced at some of these roundabouts. However, some multi-lane roundabouts have experienced a significant increase in PDO crashes. The question arises as to why some U.S. multi-lane roundabouts have substantially higher numbers of PDO crashes while other outwardly similar roundabouts do not. In this paper, we examine the hypothesis that the cause of the higher numbers of PDO crashes at some U.S. multi-lane roundabouts is driver confusion, owing to the lack of adherence to horizontal geometric safety design principles related to entry angles. We review research and explore three case studies where adherence to the safety principles related to entry angles—Phi and left view angle—contributed to improved safety.