Roundabouts : capacity and performance analysis

This report presents techniques for the analysis of capacity and performance of traffic at unsignalised roundabouts. Performance measures considered include delay, queue length, effective stop rate, proportion queued and queue clearance time. Basic principles of modelling the capacity and performance of roundabouts are discussed, the models used in the SIDRA software package are presented and data requirements are described in detail. The report extends several aspects of the SIDRA version 5.1 performance models, especially in introducing the effect of an initial queued demand in the capacity and performance models. Alternative capacity and delay models including the Australian Austroads and old NAASRA models, US Highway Capacity Manual and the UK Transport Research Laboratory models are given, model comparisons are presented including real-life cases, and various issues arising from model comparisons are discussed. Capacity model comparisons indicate that there is an overall agreement amongst various models based on gap-acceptance and other methods. However, basic comparisons as well as case studies given in the report show that various models can produce substantial differences under certain circumstances, especially under heavy and unbalanced demand conditions at multi-lane roundabouts. Recommendations for future research are presented, and a list of useful publications for those involved in research and practice related to roundabouts is given. (a)