Design Guidelines for Symmetrical Single-Lane Roundabouts Based on Intersection Sight Distance

Abstract

Checking roundabout sight distance is currently performed graphically. This paper develops analytical models for lateral clearance at symmetrical single-lane roundabouts based on intersection sight distance (ISD) and establishes related geometric design guidelines. The models are developed for two cases: an approach vehicle ahead of the yield line and before the crosswalk (Case 1), and a stationary vehicle at the yield line (Case 2). Sight distances to the respective conflicting-circulating and entering vehicles are analyzed. The maximum lateral clearance is formulated for each case using mathematical optimization. The results show that both cases of the approach and stationary vehicles are critical and therefore should be included in the ISD analysis at roundabouts. Using the developed models, design guidelines are established for the radius of the central island, entry-curve lateral clearance, exit crosswalk location, and driver’s field of peripheral vision.