An International Review of Roundabout Capacity Modelling

Roundabouts are an increasingly common form of road junction worldwide, and their effective design requires a detailed analysis of maximum vehicle throughput capacities. In this paper, the worldwide state-of-the-art in roundabout capacity modelling is examined, covering the three main methodologies on which models are based: fully-empirical, gap acceptance and simulation. It is shown that due to their limitations, each of these methodologies on their own cannot completely explain the complex behavioural and physical processes involved in roundabout entries, hence all the models require strong semi-empirical or fully-empirical bases using data obtained from their countries of origin. Differences in driver behaviour and methodologies thus result in differences in predicted capacities by the various models, and although local calibration allows some transferability, it is often limited by the availability of data or an incomplete understanding of the relationships between model parameters and capacity.