Office of Planning, Environment, & Realty (HEP)
Most two-lane streets in urban areas operate well below their uncontrolled capacity, so delay relationships for this type of facility are not critical to a forecast. Nonetheless, it is possible to make a simple change to the BPR formula (or a similar relationship) to obtain better estimates of delay.
With no opposing volume, the HCM states the capacity to be 2000 pcph. However, the capacity of a subject direction on a two-lane road depends upon its opposing volume. With a 50/50 directional split, the capacity drops to 1400 pcph. The HCM does not indicate whether this dependence on directional split holds for urban streets.
where Vs is the volume in the subject direction, Vo is the volume in the opposing direction, and τ is an empirical constant. The adjusted volume, Va, would then be used in the BPR formula when finding the volume-to-capacity ratio. The capacity would be taken to be slightly less than 2000 pcph (appropriately adjusted for heavy vehicles, terrain, narrow lanes, restricted-width shoulders, and other local circumstances).
Based on Table 8-4 in the HCM, a value of τ = 0.4 is approximately correct for rural roads. Further research is required to properly determine this constant for urban streets.