An analytical method for estimating roundabout entry-lane capacity and performance measures is presented. The method is based on modelling the gap-acceptance process that takes place in real-life roundabout operation. Unlike past studies that treated roudabouts as a series of t-junctions, the method presented here allows for approach-flow interactions. A factor is used to adjust the basic gap-acceptance capacity for the effects of origin-destination pattern and the queuing characteristics of the approach flows. Circulating stream characteristics are determined considering the approach-lane use characteristics of the traffic streams that constitute the circulating flow. The modelling of interactions among approach flows is important, especially in heavy and unbalanced demand flow cases. Ignoring approach-flow interactions can cause serious overestimation of capacity and underestimation of delays and queue lengths, especially for multi-lane roundabouts with unbalanced flow patterns. This is demonstrated through a case study that compares the results from the methods with and without approach-flow interactions. Formulae are presented for the estimation of stop-line delay, queue length, proportion queued, queue clearance time, and queue move-up rate. These formulae also allow for the effects of any initial queued demand at the start of the analysis period. The difference between the cycle-average queue and the average back of queue is emphasised. (A)
Abstract