EFFECT OF GEOMETRY ON THE TRAFFIC CAPACITY OF ROUNDABOUTS

Abstract

Roundabouts have been used as a traffic regulating measure worldwide. They are proved to be better than any other type of uncontrolled intersections. Efficiently designed roundabouts can handle traffic very smoothly without causing significant delay. There has been some controversy about capacity estimates of roundabouts in many countries. Some countries consider gap acceptance theory to estimate entry capacity of a roundabout while other considers only geometric parameters. The present study considers the combined effect of gap acceptance behavior and geometrical parameters on the entry capacity of a roundabout. This study explains how geometric parameters of the roundabout affect gap acceptance and entry capacity. For the study, nine roundabouts were selected from Chandigarh and Pune. The data needed was traffic flow and geometry of the roundabouts. Geometric parameters were measured manually with measuring tape while traffic data was collected by means of videography. Critical gaps for different types of vehicles are estimated by Modified Raff's method. Based on the study, empirical relations are suggested for the estimation of critical gaps for different types of vehicles using circulating flow and central island diameter. Entry capacities are estimated for each approach of the roundabouts using three different methods namely, HCM 2000, UK model and German model. A proposed method (Das 2007) was also used for estimating entry capacity which directly gives entry capacity for an approach from a graph between entry flow and circulating flow for that approach. The results given by this method are quite comparable to German model. In this study, an empirical relation is suggested for estimating entry capacity of roundabouts using circulating flow and geometric parameters (Central island diameter, number of entry lanes and number of circulating lanes) of the roundabouts. The suggested empirical relation is also validated by using data at another roundabout in Pune city. The validation is successful and relation is quite general for four legged roundabouts. Keywords: Roundabout, Entry capacity, Critical gap, geometry, circulating flow.