EFFECT OF VEHICLE SIZE ON SATURATION FLOW . AT SIGNALISED INTERSECTIONS

Abstract

The intersection is a place where two or more roads join or cross one another at the same or different elevation. In developing countries like India, crossing of two roads at same level is a common feature because latter is very expensive. Design of an at grade intersection under mixed traffic flow conditions to handle the crossing traffic at satisfactory level of service is the biggest problem encountered by traffic engineers. Traffic conditions that affect the saturation- flow at signalised intersection are the proportions of different types of vehicles and turning movements in the traffic stream. Thus greater the heterogeneity of vehicles in traffic stream, more is the number of obstructions in the traffic flow. The presence of vehicles of different static and dynamic characteristics has an adverse effect on traffic stream at intersections. The impact is not only due to their large size and inferior operating capabilities, but also due to psychological impact on the drivers of those vehicles which contributes to a reduction in capacity. In the present research an attempt has been made to evaluate the effect of various vehicle categories on saturation flow. A conceptual model was developed and validated for determining equivalency factors. Data collected at series of signalised intersections in Delhi have been analysed. Equations showing effect of proportion of different categories- of vehicles on its PCU values were developed. A mathematical relationship between saturation flow and approach width was also developed. To neutralise the effect of approach width, saturation flows calculated at different approaches were reduced on per lane width basis. The individual as well as combined effects of all vehicle categories on saturation flow were assessed. The equations and curves showing variation in saturation flow with proportion of vehicles of different categories in traffic stream were also developed. It is recommended that further work involving large data, intersections of varying geometrics and wider vehicular classification can be taken up in stages to draw certain guidelines for planning, design and improvements of the signalised intersections. t