Modelling saturation flow rate and right turn adjustment factor using area occupancy concept

Abstract

Saturation flow rate is the most important parameter required for the capacity analysis of signalised intersections. Under homogeneous traffic conditions, the concept of constant queue discharge headway is adopted for the estimation of saturation flow. But that concept could not be adopted under heterogeneous traffic conditions due to absence of lane discipline and wide variation in static and dynamic characteristics of vehicles. The variability in vehicle mix under heterogeneous traffic is usually accounted for in the estimation of saturation flow by using the widely accepted measure, Passenger Car Unit (PCU). But estimation of individual PCU values for different categories of vehicles under heterogeneous traffic conditions is a complex task due to its dependency on multitude of site specific factors. Hence in this study saturation flow in PCU/h is estimated with out resorting to individual PCU factors. Area occupancy, a surrogate measure of density and a measure of proportion of time vehicle area is present in observation region, is taken as the basis for the estimation of saturation flow rate in PCU/h. Area occupancy of the heterogeneous traffic stream during saturated green time is equated to standard car occupancy and the number of standard car spaces are determined. Saturation flow in PCU/h is then estimated based on number of standard car spaces. A mathematical model is developed for the prediction of saturation flow for approaches that satisfy the base conditions set in the present study. Effect of right turn movements on saturation flow rate is also explored. Turning vehicles tend to reduce the departure efficiency of through moving vehicles due to the turning manoeuvre in combined movements. Adjustment factor on saturation flow rate is proposed to account for the influence of right turn traffic that is given right of way in same phase along with through traffic.