PASSENGER CAR EQUIVALENTS FOR TRUCKS ON LEVEL FREEWAY SEGMENTS

The term passenger car equivalent (PCE) was introduced in the 1965 Highway Capacity Manual. Since 1965, considerable research effort has been directed toward the estimation of PCE values for various roadway types. However, at present, there is neither a commonly accepted nor clearly defined theoreical basis for the concept of passenger car equivalency. Two components of a theoretical basis for equivalency are defined in this paper: (a) that the basis for equivalence should be the parameters used to define level of service for the roadway type in question, and (b) that the PCE formulation should be expressed in terms of variables that reflect the relative importance of three factors that contribute to the overall effect of trucks on the roadway type. The three factors are: (a) trucks are larger than passenger cars, (b) trucks have operating capabilities that are inferior to those of passenger cars, and (c) trucks have a physical impact on nearby vehicles and a psychological impact on the drivers of those vehicles. The two components of the theoretical basis were used to evaluate the merits of three approaches to estimating PCEs for level freeway segments: (a) the constant volume-to-capaciy ratio approach, (b) the equal-density approach, and (c) the spatial headway approach. It was concluded that the spatial headway approach was appropriate for level, basic freeway segments, and a PCE formulation expressed in terms of headway measurements was derived.