Effect of Traffic Composition on Capacity of Multilane Highways

Mixed traffic has always been a major concern in all studies related to estimation of capacity of a highway in developing nations like India. The problem of mixed traffic is generally circumvented by making use of passenger car equivalency factors for different categories of vehicles and converting them into equivalent number of Passenger Cars Units (PCU). However, capacity is also required in terms of vehicles per hour, particularly for quick estimation of volume-to-capacity ratio on a highway. The present study investigates the effect of traffic mix on capacity of four-lane, six-lane divided highways in India. All vehicles on a highway are divided into five categories namely standard car, big car, heavy vehicle, motorized three-wheelers and motorized two-wheelers, and effect of each vehicle type on capacity of the highway is evaluated. The VISSIM software is calibrated and used to generate the traffic operations based on field data using capacity as the measure of performance. Speed-flow curves are developed to find simulated capacity values for different combinations of standard car and one of the remaining four types of vehicles in the traffic stream. The results are finally combined to propose the generalized equations to determine capacity when all or few of the five categories of vehicles are simultaneously present on a highway.