On the Treatment of Trucks in Roundabout Analyses

Trucks are not always addressed explicitly in planning studies and operational analyses, even though they are a very important part of the nation's economic engine. Most planning studies account for trucks as a percentage of the total traffic flow. The Highway Capacity Manual (HCM) converts trucks into autos through the use of a passenger car equivalency (PCE) factor. If the PCE is 2, then a truck is assumed to be equivalent to two autos. However, trucks are beginning to receive more attention, including that in the context of capacity assessments. A recent research effort focused on developing a level-of-service assessment for trucks and for enhancing the way that the impact of trucks is taken into account on freeways, arterials, and roundabouts. This paper focuses on the roundabout aspects of that research. The three roundabout-related objectives in the project were (a) to refine the PCE values used to convert trucks into auto equivalents for the flow on roundabout approaches, (b) to recalibrate the capacity equation so that it more appropriately accounts for trucks, and (c) to develop ways to estimate truck speeds so that travel times through the roundabout could be computed. The results of these analyses are presented as well as the methodology and data employed.