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Abstract
Highway Capacity Manual (HCM) (2000) states that the capacity of a two-lane road is nearly independent of the directional split of traffic. However, a study conducted on two-lane roads in India has shown that capacity reduces as the split moves away from 50 / 50. The capacity of a two-lane road at even split in two directions is estimated as 2920 passenger car units per hour (pcu/h), which is less than the value specified in HCM (2000). The effect of slow-moving vehicles is also evaluated and it is found that capacity decreases as the proportion of slow-moving vehicles in the traffic stream increases.
INTRODUCTION
Two-lane highways are important because they make up the predominant length of the highway systems in every country. There are approximately 3 million miles of two-lane highways in US. The bulk of major road network in India is also two lane. Two-lane roads have typical characteristics of traffic flow where one lane is marked for use of vehicles in each direction. Lane changing and overtaking is possible only in the face of on-coming traffic in the opposite lane. Overtaking demand increases rapidly as traffic volume increases, while overtaking opportunities decline as traffic volume in the opposite lane increases. It results in formation of platoons in the traffic stream, which are subjected to delays because of the inability to overtake. The factors which affect the capacity of a two-lane rural highway include lane width, shoulder condition, grade, curvature, directional split, traffic mix and percentage of slow-moving vehicles. HCM (2000) indicates that the capacity of a two-lane road is nearly independent of the directional distribution of traffic. However, this study shows that the capacity of a two-lane road under mixed traffic condition is a function of the split of traffic in two direction.
The traffic stream in India consists of two distinct categories of vehicles, namely fast-moving (motorised) vehicles and slow-moving (non-motorised) vehicles. The static and dynamic characteristics of these two types of vehicles vary widely. The speed difference between a fast-moving and a slow-moving vehicle could be as high as 60km/h. A slow-moving vehicle with a speed of 10-12 km/h causes disturbance of traffic operation and reduces the capacity of the road. Non-motorised traffic is quite common on...