Critical Gap as a Function of Waiting Time in Determining Roundabout Capacity

This paper presents a study of two important roundabout performance issues: (1) the impact of the waiting time of drivers on the approach leg, on the critical gap they accept before entering a roundabout and (2) the entry capacity of the approach. The main hypothesis is that an increase in drivers’ average waiting times reduces the average accepted gap and, consequently, the critical gap. A model for a change in the critical gap at busy roundabouts, with long approach queues and long waiting times, is provided. The model for a change in the critical gap was found to be logistic; i.e., it had an s-shape between a maximum and a minimum critical gap. The parameters of each site studied in the present investigation were found to correlate in regard to geometric complexity and traffic intensity. The parameters were the outside diameter, pedestrian volume, and circulating flow of vehicles around the central island. A reduction in the critical gap because of increased waiting times results in a significant increase in entry capacity, which is particularly significant in high circulating volumes. A model for the entry capacity that depends on the circulating flow, the outside diameter, and the critical gaps (which, in turn, depend on the waiting times) was calibrated and presented. This model was found to be sensitive in high circulating volumes to the critical gap; and in low circulating volumes, to the geometry, represented by the outside diameter. The use of this model is recommended when estimating the entry capacity of either new or existing roundabouts.