PARAMETERS OF SPEED-DISTRIBUTION CURVE UNDER MIXED TRAFFIC CONDITIONS

Abstract

Road transport has been developing into, an increasingly important mode of transport of men and materials. There are variety of vehicles moving on roads in India and other developing countries. Mixed traffic has been the interest of traffic researchers and some useful studies have been done in different parts of the world to characterise and understand mixed traffic flow behaviour. The term "mixed" in relation to traffic flow is somewhat analogous to an "impurity". The reason is very simple since the entire flow is characterised in terms of "passenger cars", any thing other than a "passenger car" has always created a "mix" and subsequently its own characterics problems. Theproblems due to mixed traffic flows start from the planning stage and involve themselves through various stages of design, operation and control in continuous chain action. Highway Capacity Manual (HCM, 1985) and other related standards suggest 85th percentile speed as a design speed of highway for uniform traffic condition. Design speed of an highway has direct effect on its capacity. In a mixed traffic situation where slow and fast moving vehicles, do not have separate lanes, the criteria for selecting design speed should be based on the most dominating speed rather than conventional 85th percentile speed. In the present study, the data was collected at few mid block sections of urban roads in Delhi. Speed- distribution curves were drawn for each section and their characteristic parameters were established. The slope of the curve was found to be dependant on proportions of heavy and slow moving vehicles in the traffic stream. The most dominant speed of highway was calculated for each set of observation and it was found that it is well below the 85th percentile which is conventionally taken as the design speed. The 77th, 76th and 79th percentile speed were found to be the most dominating speed on 3-highways selected for this study. It is recommended that 80th percentile speed should be considered for design of highways carry mixed traffic of the type prevailing in India.