Roundabout intersections, increasingly used in the U.S., sometimes incorporate slip lanes to facilitate right-turning traffic flow and reduce delay, thereby increasing capacity and safety. Performance of a single-lane roundabout with an adjacent slip lane is modelled with the SIDRA Intersection analysis tool for three types of slip lane: free-flow, yield, and stop, and is compared to having no slip lane. The gap acceptance-based assessment considers four experimental traffic percentage distribution matrices representing flow scenarios. SIDRA results confirm that average delay and circulating conflict volumes in a roundabout with a slip lane are related exponentially to slip lane volumes. A free-flow slip lane exit type helps to reduce total average delay in the roundabout and the slip lane approach. Both yield and stop slip lane exit types also reduce roundabout total average delay but to a lesser degree than a free-flow slip lane. Finally, theoretical capacity threshold values for slip lane volumes are estimated to range from 150 to 350 vehicles per hour for traffic volume distribution scenarios.
AL-GHANDOUR, M.N., RASDORF, W.J., WILLIAMS, B.M. and SCHROEDER, B.J. (2011). Analysis of Single-Lane Roundabout Slip Lanes using SIDRA. Proceedings of the First Transportation and Development Congress, Chicago, IL, USA, Mar 2011, ASCE, pp 1235-1244.