Comparative study on the capacity of a signalised roundabout

Abstract

Traffic signal control at roundabouts is proved to be effective in addressing heavy traffic. However, the applicability of a signalised roundabout, especially compared with a signalised intersection, in terms of capacity has not been investigated. For signalised roundabouts, unsignalised roundabout capacity models including gap-acceptance theories and regression models are not applicable. This study first introduces a shockwave-based model to capture the impacts of signals at circulatory lanes on queuing and clearance time of left-turning vehicles. On this basis, a general signal timing model and a capacity model are developed. A sensitivity analysis is then elaborated to test how temporal and spatial parameters affect the capacity of a signalised roundabout. Capacity comparisons are made among signalised roundabouts, unsignalised roundabouts and signalised intersections. The results show that (i) under high traffic demand, a signalised roundabout outperforms an unsignalised roundabout; (ii) a signalised roundabout achieve higher capacity than a signalised intersection does under certain condition, (iii) but a signalised intersection outperforms a signalised roundabout when optimal cycle length is relatively short (65 seconds in this study), (iv) capacity of a signalised roundabout is very sensitive to the variation of signal parameters, hence care should be taken to design the parameters to ensure high capacity.