Estimation and Comparison of Volume Delay Functions for Arterials and Freeway HOV and General Purpose Lanes

Maricopa Association of Governments (MAG) developed and implemented new sets of volume delay functions and road network capacities. Some innovative approaches were implemented in utilizing the Greater Phoenix Area regional travel time and speed survey and other regional data sources. This paper describes methodology for the functional form selection, estimation and calibration of the delay functions and capacities. The study estimated parameters for several functional forms including the Bureau of Public Roads (BPR), Spiess conical delay, and Akcelik functions. Curves were fit for both high occupancy vehicle (HOV) and general purpose freeway lanes as well as for arterials for several area types. With the proper choice of parameter values, the BPR function generally offered comparable or better goodness-of-fit to the data than the other functions, although the parameters varied significantly by facility type. The BPR functions were then used to perform user equilibrium traffic assignments and some final parameter adjustments were made. Assignments using the final functions produced more accurate speeds and smaller modeled volume errors on screenlines versus observed traffic counts.