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.
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. 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.