Comparison of Capacity Models for Two-Lane Roundabouts

Abstract

Multilane roundabout capacity models are presented and contrasted in the context of a case study. A two-lane roundabout in Copenhagen, Denmark, was investigated, and data were collected that enable the estimation of critical gaps, follow-on times, entry capacity, and delay. The primary objective of this research was to evaluate the need for more complex capacity models than currently exist in order to properly represent driver gap-acceptance behavior at multilane roundabouts. The complexity arises when drivers are assumed to simultaneously accept pairs of critical gaps in the outer and inner circulating lanes before they enter the roundabout. This approach requires that circulating-lane headways and critical gaps be evaluated independently and not be superposed in a single traffic stream as most current capacity guides assume. The approach also implies that circulating-lane volume allocation can have a considerable impact on entry capacity. The field results indicated that indeed there were differences in the gap-acceptance behavior for drivers entering in the right approach lane, with critical gaps estimated at 3.68 s and 4.49 s for the inner and outer circulating lane, respectively, at the test site. In contrast, drivers entering in the left approach lane exhibited no difference in critical gaps among lanes (4.64 s for the inner lane and 4.68 s for the outer lane). Finally, the lane allocation of circulating flow did have a significant impact on capacity, particularly at large circulating-flow rates. This finding implies that the origin and destination of the flow constituting the circulating traffic must be accounted for in estimating capacity. Even small errors in capacity estimation were found to have a significant impact on delay and level of service.