Desired Time Gap and Time Headway in Steady-State Car-Following on Two-Lane Roads

Two continuous statistical distribution models, gamma and lognormal, are proposed for desired time gap (TX) and time headway (T) of drivers in a steady car-following state on two-lane roads under mixed traffic conditions. A simulation program was developed for traffic movement on two-lane roads incorporating actual behavior of mixed traffic in terms of speed, placement, arrival pattern, and overtaking behavior. The program was run at 50/50 directional distribution with varying traffic composition and volume level. Headway data obtained from simulation runs are analyzed to develop a relationship between the desired time gap (TX) and the speed of the vehicles for five categories of vehicles, namely, car, heavy vehicle, motorized two-wheeler, three-wheeler, and tractor. It is observed that the desired time gap of a two-wheeler is the minimum of all other categories of vehicles. It is due to their better maneuverability, which allows them to maintain shorter gaps. The desired time gap for tractors is the maximum of all vehicles, which is due to their low acceleration capability and poor braking efficiency. These relationships will be extremely useful in developing the microsimulation models of traffic flow on two-lane highways.