Roundabouts with Unbalanced Flow Patterns

Abstract

An improved understanding of the effect of origin-destination demand pattern of traffic on roundabout capacity and level of service will help towards designing new roundabouts that will cope with future increases in demand levels and solving any problems resulting from unbalanced flow patterns at existing roundabouts. Case studies are presented to show that roundabout capacity and level of service depend not only on the circulating flow rate but also the characteristics of approach flows contributing to the circulating flow. The amount of queuing on the approach road, circulating lane use, priority sharing and priority emphasis are the factors that need to be taken into account. Dominant circulating flows that originate mostly from a single approach with high levels of queuing and unequal lane use (with most vehicles in one lane), cause priority emphasis and reduce the entry capacity significantly. This is evident from the use of part-time metering signals under peak demand conditions in order to alleviate the problem of excessive delay and queuing by creating gaps in the circulating stream. The Australian roundabout and traffic signal guides acknowledge the problem and discuss the use of metering signals.