Performance of roundabouts under heavy demand conditions

The changes introduced in SIDRA 4.1 for improved prediction of capacity and performance of roundabouts under heavy demand conditions are described. The new method, developed in response to problems observed at real-life roundabouts, predicts generally less optimistic results compared with the current AUSTROADS roundabout guide, particularly for multi-lane roundabouts with high circulating flow rates and unbalanced flow patterns. The new method takes into account the effects of the origin-destination pattern, lane usage and queueing characteristics of approach flows. As such, it is the first analytical method that models interactions among approach flows, rather than treating the roundabout as a series of independent T-junctions. Various other changes have been introduced to the modelling of critical gap and follow-up headways to prevent overestimation of capacities. A further and significant change is to decrease the follow-up headway in the case of heavy entry flows against low circulating flows. The implications of the new method in terms of the capacity and performance of roundabouts are discussed. The application of the new method to a case study from the AUSTROADS roundabout guide is also presented.