Impact of Capacity Definition on the HCM-6 Passenger Car Equivalent Values

In the current version (Version 6) of the Highway Capacity Manual (HCM), equal-capacity passenger car equivalencies (EC-PCEs) are used to account for the effect of trucks for capacity analyses. The EC-PCEs for freeway segments were estimated using a microsimulation-based methodology in which the capacity of different traffic scenarios was modeled. However, the current EC-PCEs were estimated under assumptions atypical of past HCM releases, including a new definition of capacity and a new data aggregation level. It is important to assess to what extent these assumptions may affect the passenger car equivalencies (PCE) values published in the HCM-6. Consequently, this paper compares the HCM-6 PCEs, and associated capacity adjustment factors (CAF), with values developed using the HCM-6 EC-PCE methodology with historic HCM assumptions. It was found the EC-PCE values from the proposed approach were, on average, 18.2% greater than those published in the HCM-6. This finding suggests the values published in the HCM-6 could underestimate the effect of trucks in capacity and level of service analyses.