Office of Planning, Environment, & Realty (HEP)
The BPR function fits the various delay/volume relations in the HCM with good consistency. If only one curve can be chosen, the BPR function is preferred to Spiess' and Overgaard's.
Capacity is the most important variable when estimating volumes on congested highways. Since the definitions of levels of service vary greatly by facility type, "capacity" in delay/volume functions should be set at LOS E, ultimate capacity. Design capacity should be phased-out as a variable in delay/volume functions.
Because of the large number of factors affecting capacity of uncontrolled road segments, capacity should be separately determined for each link. The Highway Capacity Manual provides procedures for most types of facilities, and these procedures should be followed.
If only one set of parameters can be chosen for the BPR function, then the volume-to-capacity multiplier, α, should be approximately 0.83 and the volume-to-capacity exponent, β, should be approximately 5.5.
Additional research is needed on capacity of two-lane streets in urban areas.
Travel forecasting software should contain procedures, similar to those in the HCM, in order to achieve more precise estimates of capacity and delay at intersections.
In the absence of such software, planners can still improve their forecasts while calibrating their networks. Planners should adopt one of the methods presented in this report to better specify capacity at intersection approaches.
During calibration, planners need to achieve consistency between their assigned volumes and the nature of traffic control at intersections. This can be done by referencing signal warrants from the Manual on Uniform Traffic Control Devices or by comparing total delay from alternative traffic control strategies. Planners need not consider the possibility of all-way stop controlled intersections, unless this form of traffic control is required for purposes other than minimizing delay.
Network calibration, as now practiced by planners, appears to be a means of overcoming deficiencies in existing delay/volume relationships. It is important that the same calibration process, which is applied to the base network, also be applied to future-year networks. Specifically, planners need make sure that their values of capacity are consistent with the distribution of traffic at intersections, at weaving sections, and at two-lane roads. It is not possible to assume that values of capacity set for the base-year network also hold for future-year networks.