Determination of Passenger-Car Units on Two-Lane Intercity Highways under Heterogeneous Traffic Conditions

The introduction of the passenger car unit (PCU) concept by the 1965 edition of the Highway Capacity Manual (HCM) enabled researchers around the world to deal with mixed traffic flow. Though some researchers have tried to show the dynamic nature of PCU factors, most of the studies in the world, as well as the latest standards being followed, still provide static PCU values for various types of vehicle categories. The present study demonstrates the dynamic nature of PCU factors on two-lane intercity highways under highly heterogeneous traffic composition. They are estimated based on speed and size of vehicle type in the traffic stream with respect to a standard passenger car. Simultaneous equations are formulated to calculate the speeds of different types of vehicles for a given traffic composition and traffic volume on the road, which lead to the determination of PCU factors. While many plots are made to describe the effect of traffic composition and volume on PCU of a vehicle type, it is felt that they are not enough to explain the complete variation. Therefore, the concept of a stream equivalency factor is suggested to convert a heterogeneous traffic stream into a homogeneous stream consisting of passenger cars. A simple linear equation is developed using data from three sites to calculate this factor for a known traffic volume and proportional composition of a traffic stream. Field data from a fourth site are used to validate the proposed model. The proposed model is quite simple to use and would avoid estimation of PCU factors for individual types of vehicles on a two-lane intercity highway. (C) 2015 American Society of Civil Engineers.