Field-Derived Freeway Passenger Car Equivalents for Congested Conditions

Abstract

Heavy-vehicle (HV) impact on freeway operations in the Highway Capacity Manual 2010 is evaluated on the basis of values for passenger car equivalents (PCE) for buses–trucks and recreational vehicles. These values were calibrated for relatively uncongested freeway conditions [Levels of Service (LOS) A through C] because inadequate field data on HV behavior under congested conditions were available; furthermore, field-collected headway information was based on the average headway for vehicles in a particular class, regardless of the type of vehicle that they were following. The goal of this paper was to collect and analyze freeway field data on HV headways with an emphasis on lower speeds typically associated with an LOS E (capacity) or F (forced-flow conditions). The effect that a leading vehicle has on headways was captured and analyzed for 10 leading–following vehicle pair types containing at least one passenger car (for example, buses followed by passenger cars). Headway statistics were analyzed for nine speed ranges (up to 20 mph, 20 to 25 mph, 25 to 30 mph, 30 to 35 mph, 35 to 40 mph, 40 to 45 mph, 45 to 50 mph, 50 to 55 mph, and over 55 mph) and 10 vehicle pair types. Values for PCEs were derived for each speed range on the basis of average headway for a specific vehicle pair type divided by the average headway between passenger cars. Leading and following vehicle classes and speeds were found to have a significant influence on headways and values for PCEs.

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