1 Examining the effect of heavy vehicles on traffic flow during congestion by Al-Kaisy, Ahmed, Jung, Younghan Abstract An investigation into the effect of heavy vehicles on traffic flow during congestion is presented in this paper. Several factors that are thought of as determinants of this effect were considered in the investigation. Empirical data and microscopic traffic simulation were used in the analysis. A passenger car equivalency factor derived from queue discharge flow was used as an indicator of heavy vehicles' effect. While study results suggest some similarities between free-flow and congested traffic regimes concerning heavy vehicles' effect, some important differences exist due to the different mechanisms that govern heavy vehicles' performance in the two regimes. Also, lane-use restriction and the location of bottleneck relative to upgrades were found to have considerable influence on heavy vehicles' effect during congestion. INTRODUCTION Heavy vehicles (HVs) are known for their significant effect on traffic due to their larger dimensions (in general) and inferior performance compared with an average automobile. These vehicles include trucks, buses, and recreational vehicles, with each category having a wide variety of size, power, and design concepts. Historically, the Highway Capacity Manual (HCM) procedures (TRB 2000) have treated heavy vehicles' effect through the use of Passenger Car Equivalents (PCEs). Using those PCEs, a non-homogeneous mix of vehicles in a traffic stream can be expressed in a standardised unit of traffic, i.e. Passenger Car (PC). While the PCE factors provided by the HCM were based on free- flow conditions, they have been used (unknowingly) by transportation professionals for all traffic conditions, i.e. congested and non-congested operations. Recent empirical observations suggest that heavy vehicles' effect is significantly greater during congestion and queuing operations (compared with free-flow operations) and that the use of the HCMPCE factors for congested facilities may involve a considerable amount of error (Al-Kaisy et al. 2002). Specifically, the acceleration and deceleration cycles, a situation normally experienced during congestion or stop-and- go conditions, are expected to impose an extra limitation on the performance of heavy vehicles, and in particular on their acceleration away from the front end of the queue that had been moving slowly. The literature review found no other study that investigates the heavy vehicles' effect during congestion. The current research involves an investigation of several important factors on the effect of heavy vehicles during congestion using empirical data and traffic simulation. These factors involve grade, grade length, percentage of heavy vehicles, lane-use restriction by vehicle type, and the location of bottleneck with respect to grade. This investigation is deemed very important due to the fact that recurrent and non- recurrent bottlenecks have become commonplace on urban highways, and many traffic engineers need to deal with congested facilities on a regular basis. Examples of situations with bottleneck operations mainly include recurrent congestion on major urban freeways during peak hours, lane closures at reconstruction and maintenance zones, lane closures due to traffic accidents/incidents, and traffic situations during major events. Further, as noted above, no information exists in the literature concerning the effect of heavy vehicles during congestion. This