LINK PERFORMANCE FUNCTIONS FOR URBAN FREEWAYS WITH ASYMMETRIC CAR-TRUCK INTERACTIONS

Link performance functions, which capture the relationship between travel time per unit distance and traffic volume per unit time on the links of a network, constitute an essential element in the equilibrium assignment of traffic flows to congested transportation networks. Results are presented of the empirical development and calibration of performance functions that capture the dependence of travel time on the respective volumes of passenger cars and trucks sharing the physical right-of-way on urban freeway sections. The data used for model calibration, individual vehicle trajectories on urban freeway sections, originally collected for FHWA, are developed from a secondary data base. Despite the data limitations, useful relations applicable to a broad range of freeway traffic conditions are developed that yield insights into the effect of trucks on freeway performance. Two types of functions are presented: (a) linear functions over the range of operating volumes extending up to 1,300 vehicles per hour per lane and (b) nonlinear functions, based on the widely used Bureau of Public Roads form, over the full range of flow values. A secondary analysis of the relation between truck and car average travel times is also discussed. These functions are intended for use in network equilibrium studies requiring the assignment of explicit car and truck flows and therefore involving asymmetric interactions between these vehicle classes (or equivalently between links). Such problems arise in the context of the evaluation of truck-related highway improvements, which is a problem of current interest to highway agencies.