Simulation Study of Truck Traffic at Single-Lane Roundabouts with and without Slip Lanes

Abstract

As transportation planners and engineers design useful and effective roundabouts, these professionals are challenged by the need to accommodate safely truck traffic and high truck volumes in particular. Delay is a major challenge with truck traffic, especially with returning left-turning trucks. The delay performance of single-lane roundabouts with an adjacent slip lane for right turns was considered under various truck traffic percentages and two slip lane exit types (free flow and yield). A microsimulation assessment compared four percentages of right-turn truck traffic: 0% (no trucks), 5%, 45%, and 80%. Results indicate that the average delay of a roundabout with a slip lane under various truck traffic percentages is a nonlinear relationship with slip lane volumes and is sensitive to changes in truck traffic percentages before oversaturation is reached. As expected, results indicate that a free-flow slip lane exit type significantly reduces total average delay in roundabouts compared with having no slip lane with truck traffic. Yield slip lane exit types also reduced total average delay from truck traffic in roundabouts, but to a lesser degree than free-flow slip lane exit types. At higher truck traffic volumes, overall average roundabout delay decreased 15% (estimated VISSIM 95% confidence interval of reduction estimated between —16% and —2%) with a free-flow slip lane exit type. Finally, returning left-turn trucks increased total roundabout average delay significantly, by 64%.