Impact of trucks on arterial LOS and freeway work zone capability: part A

Large trucks have considerably different size and performance characteristics than passenger cars. Consequently, these trucks can have a significant impact on traffic operations. It is therefore essential to properly account for this impact in the traffic operations analysis in order to reflect operational quality of the roadway as accurately as possible. Signalized intersections are one roadway facility that can be particularly sensitive to the presence of commercial truck traffic. The most common methods used for the analysis of signalized intersections is contained in the Highway Capacity Manual (HCM). In this method, the base saturation flow rate of the signalized intersection is defined in units of passenger cars per hour green lane (pc/hg/In). To account for the presence of large trucks in the traffic stream, the HCM includes Passenger Car Equivalency (PCE) value. In the current edition of the HCM, a PCE value of 2.0 is applied to all large trucks, with no distinction between different size of trucks. Some transportation professionals have questioned the validity of this PCE value recommended by the HCM. They are concerned that the impact of trucks at signalized intersections is being under-estimated. If this is the case, then capacity is being over-estimated and intersections are not being adequately designed. The primary objective of this research was to determine appropriate truck PCE values to apply for signalized intersection analysis. These PCE values were classified by three different categories of truck size and performances. Additionally, a general PCE value with only one truck category was developed for planning purposes and/or a less detailed analysis. The development of the PCE values was based on the relative headway concept, as defined in the HCM. The results of this study are based primarily on data generated from a custom simulation program. However, a considerable amount of field data was collected for the purpose of simulation calibration. The PCE values determined from this study are 1.8, 2.2, and 2.8 for small trucks, medium trucks, and large trucks, respectively. Additionally, and equation was developed to calculate start-up lost time that accounts for the impact of trucks at the front of the queue, as opposed to standard 2.0 seconds recommended by the HCM. Furthermore, based on the field data collected, it was found that the base saturation flow rate value of 1900 pc/hg/ln recommended by the HCM appears to be quite optimistic.