ROUNDABOUTS WITH UNBALANCED FLOW PATTERNS

This paper describes how complex interactions among the geometry, driver behavior, traffic stream and control factors determine the roundabout capacity and level of service. With increased use of modern roundabouts, 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. Unbalanced flows may not be a problem when the overall demand level is low but appear with traffic growth even at medium demand levels. Modeling of traffic demand patter is important in optimizing the roundabout geometry including approach and circulating lane use. Case studies are presented to show that roundabout geometry including and level of service depend not only on 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 lane 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 lane use of part-time mattering signals under peak demand conditions in order to alleviate the problem of excessive delay and queuing by creating gaps in the circulating stream. This is a cost-effective measure to avoid the need for a fully signalized intersection treatment. The Australian roundabout and traffic signal guides acknowledge the problem and discuss the use of metering signals.