CAPACITY ANALYSIS ON MULTILANE HIGHWAYS IN MIXED TRAFFIC CONDITIONS

Abstract

Traffic conditions on Indian highways are highly heterogeneous due to variety of vehicles with different static and dynamic characteristics sharing and operating on common roadway width without following proper lane-discipline. The expression of capacity in terms of number of vehicles passing through a given section on roadway in any time interval will be meaningless in such situations unless volume information is accompanied by the traffic composition. Capacity of highway would depend on the average speed of vehicles and composition of traffic stream. The traffic volume measured at a section is required to be converted into equivalent numbers of standard vehicles (passenger cars) to express capacity of a roadway section. Most of the studies reported in literature are from US and other developed nations where homogeneity of traffic stream and lane discipline of drivers are two important characters of traffic flow. Very few attempts are made in India to evaluate capacity standards for multilane highways under mixed traffic and untidy behavior of drivers. The present research was taken up with the objective of determining capacity of four-lane, six-lane and eight-lane divided inter urban highways in India, and to quantify the effect of traffic composition on capacity. Field data collected on sections of multilane interurban highways are analyzed to determine the basic parameters of traffic flow. All vehicles are divided into five categories such as Standard car (CS), Big utility car (CB), Heavy vehicle (HV), Motorized two-wheelers (2W) and Motorized three-wheelers (3W), and their speed and acceleration characteristics are studied on three types of highways. These data are used to calibrate driver behavior parameters in VISSIM model by taking roadway capacity as the measure of effectiveness. These parameters were found to be vehicle type dependent and therefore, mathematical equations are suggested to determine their values under mixed traffic conditions. ii The calibrated VISSIM was then used to determine capacity of four-lane divided road with standard cars and one of the remaining four types of vehicles at varying proportions of the second type of vehicles in the traffic stream. Speed-flow curves are developed for each proportional combination of two vehicles to estimate the capacity of the section. Similar, procedure was used for six-lane and eight-lane roads; and mathematical equations are suggested to calculate capacity of a highway at given composition of traffic stream. Capacity of a highway was found to reduce with increasing proportion of vehicles which are larger than standard car. It however, increased with increase in the proportion of motorized two-wheelers. The data are further analyzed to determine PCU of different types of vehicles at different volume-to-capacity ratio and composition of traffic stream. These values can be used by field engineers to convert heterogeneous traffic condition into a homogenous equivalent.