Influence of Priority Violations on Entry Capacity at Roundabouts

Abstract

The gap acceptance theory, which is widely used for capacity analysis of roundabouts, assumes absolute priority to circulating vehicles over entry vehicles. Priority violations such as priority reversal and limited priority are common in heterogeneous traffic conditions and render the gap acceptance models ineffective. The frequency of priority violations is significant for quantitative analysis. The frequency of priority violations depends on roundabout geometry, traffic heterogeneity, circulating flow, and gap parameters. Analysis of the trends of data collected for 21 roundabout entries at six roundabouts in India indicated that frequency of priority violations vary based on traffic composition and circulating flow. Separate models are formulated to estimate frequencies of priority reversal and limited priority. The effect of buses on priority reversal is found to be twice when compared to that of two-wheelers, but only half as that of heavy vehicles. The Indian Highway Capacity Manual (Indo-HCM) model for roundabout capacity was modified by incorporating priority violations, and estimates of the modified model are comparable to the observed entry flow values. The influence of frequency of limited priority on roundabout capacity is found to be six times that of frequency of priority reversal.