Limitations of Passenger-Car Equivalent Derivation for Traffic Streams with More Than One Truck Type

Heavy vehicles can have a major impact on traffic streams, particularly on grades. Traditionally, these impacts have been expressed in terms of a passenger-car equivalent (PCE), which is used to convert a mixed traffic stream into a hypothetical passenger-car stream. When the stream contains more than one truck type and truck characteristics differ significantly, it would be desirable to derive PCEs for each truck type. Limitations of current methods of PCE derivation are discussed, and it is demonstrated that PCEs derived individually for each truck type account for only part of the impact caused by trucks on traffic. As a result, distortions are created in the estimation of equivalent flow rates when the traffic composition is different from the one used in the derivation of PCEs. A work-around solution based on the estimation of an aggregate PCE is discussed, and a quantitative analysis of the errors associated with the use of each type of PCE is presented. The results indicate that the errors in the estimation of equivalent flow rates are negligible for densities less than 10 veh/(km-lane) but increase significantly with the increase in density. Equivalent flow rates calculated with aggregate PCEs are not exempt from errors, especially if the traffic mix is different from the one used for the base stream, but these errors were found to be smaller than those caused by the use of PCEs derived individually for each truck type.