For several decades, the Highway Capacity Manual has been applying the concept of Passenger Car Equivalents (PCE) to capture the effects of heavy vehicles in the traffic stream. These models assume heavy vehicles have different powertrain and acceleration characteristics, and therefore one heavy vehicle would have a greater impact on traffic when compared to a passenger car.
However, the PCE factors may not be accurate when at least one of these conditions exists:
- Significant presence of trucks in the traffic stream
- A long upgrade
- A combination of both factors above
The HCM mentions the Mixed-Flow Model to address this issue in freeway basic segments and multilane highways. While the PCE approach assumes vehicle speeds are uniform across all vehicle types, the Mixed-Flow model calculates speeds for passenger cars and trucks individually, yielding more accurate results.
The required inputs for the Mixed-Flow Model include:
- Total percentage of trucks and the proportional distribution of Single-Unit Trucks (FHWA classifications 4-5) and Tractor-Trailer Trucks (FHWA classifications 6-13)
- Grade percentage (%) and length (mi)
For low percentages of trucks and mild upgrades, the results provided by the PCE methodology are comparable to the ones provided by the mixed-flow model. The HCM, however, does not provide a strict definition of what values of %HV and grade are considered significant to support a mixed-flow analysis.
Therefore, the main question remains:
Under what conditions should we consider using the mixed-flow model?
To illustrate the differences between PCE and Mixed-Flow outputs, a sample basic segment with the following characteristics is analyzed:
- Demand: 4,000 veh/h
- Free-Flow Speed: 65 mi/h
- 8% grade, with a 1-mile length
- Truck mix: 30% SUT, 70% TT