Incorporating Exiting Vehicles in Capacity Estimation at Single-Lane U.S. Roundabouts

The current model used in the U.S. to predict approach capacity at a single-lane roundabout utilizes information about entry driver behavior in relation to the circulating stream of traffic only. No procedure is currently in place for incorporating exiting vehicles in capacity estimation. Exiting vehicles have been shown to have an effect on capacity at roundabout approaches in other countries, but it is not known what effect, if any, exiting vehicles have at roundabout approaches in the U.S. The purpose of this research effort is to determine if the incorporation of exiting vehicles improves capacity estimation at a roundabout approach, and to explain capacity prediction errors through the examination of particular geometric and flow parameters that govern entry and exiting vehicle interactions. Approach capacities were estimated using HCM Equation 17-70, with and without the incorporation of exiting vehicles, and compared to measured field capacities. The findings presented in this report demonstrate that capacity estimates with exiting vehicles result in improved prediction of the actual capacity of a roundabout approach over estimates without exiting vehicles. It was determined that the parameters proportion of exiting vehicles in the major stream and the width of the splitter island provide some explanation of capacity prediction errors, but exactly how the parameters should be incorporated into the capacity prediction process needs to be further explored.