The hourly flow rate must reflect the effects of heavy vehicles, the temporal variation of traffic flow during an hour, and the characteristics of the driver population. These effects are reflected by adjusting hourly volume counts or estimates, typically reported in vehicles per hour (vph), to arrive at an equivalent passenger-car flow rate in passenger cars per hour (pcph). The equivalent passenger-car flow rate is calculated using the heavy-vehicle and peak-hour adjustment factors and is reported on a per lane basis, or in passenger cars per hour per lane...
Heavy Vehicle Adjustment Factor
Freeway traffic volumes that include a mix of vehicle types must be adjusted to an equivalent flow rate expressed in passenger cars per hour per lane. This adjustment is made using the factor fHV.
Adjustments for the presence of heavy vehicles in the traffic stream apply for three vehicle types: trucks, buses, and RVs. There is no evidence to indicate any differences in performance characteristics between the truck and bus populations on freeways, so trucks and buses are treated identically.