Study of vehicle arrival pattern and free speed characteristics on selected national highways

Abstract

Analysis and interpretation of traffic Operations on national highways reqUIre a sound understanding of the traffic flow parameters. Such traffic flow parameters are traffic arrival headways, free speed of vehicles, operating speed and speed-flow-density relationship. No significant study has yet been made to investigate these traffic parameters of Bangladeshi national highways. In this study, effort has been given to investigate the traffic arrival pattern and free speed characteristics of vehicles on two-lane two-way national highways of Bangladesh. The research study has been based on field data. Both video and manual data on the traffic flow of selected national highways have been collected for a net period of approximately twenty five hours. The analysis of collected data has been made using different statistical softwares. Analysis of traffic arrival pattern using the vehicular time headway data has revealed that the pattern follows more than one statistical distribution models for all the highways. Generally, it has been observed that the vehicles maintain close head ways when they are in following situation. It has been observed that the traffic arrival pattern on selected Bangladeshi national highways can be described by one or more of the two statistical distributions, namely, exponential and lognormal. The relevant parameters of the corresponding arrival headway distributions have been estimated. Analysis of free speed data of vehicles on a typical section of Dhaka-Aricha highway has revealed that the free speed of the commonly found vehicles(bus, minibus, truck, passenger-car, nonmotorised vehicles) follow normal distribution pattern. Corresponding parameters of the normal distribution have also been estimated. From regression analysis, it has been found that free speed of vehicles depends on the pavement and shoulder width. In a pavement width range of 5.8m to 7.5m, free speed of commonly found motorised vehicles increases in a range of 7.25 kmph to 10.29 km/ph for each metre of pavement widening for flat, straight and disturbance free highway section. The analysis has also revealed that free speed of vehicles increases with the increase in shoulder width except for the case of bus(the reason of which has been identified as a local phenomenon on a particular highway).