Nebraska LTAP
Date of this Version
2-2017
Document Type
Article
Citation
Zhou, J., Rilett, L., and Jones, E. (2017), Assessing Passenger Car Equivalency Factors for High Truck Percentages. NDOT Research Report SPR-P1(15) M023.
Abstract
The Passenger Car Equivalent (PCE) values in the Highway Capacity Manual (HCM) 2010 might not be valid for western Nebraska freeway conditions. This is because: 1) Interstate 80 (I-80) experiences high truck percentages (25% to 60%), while the HCM provides PCE values up to 25% of the truck percentage; 2) the average speed of trucks are observed lower than passenger cars, which is incompatible with the HCM assumption that the free speed of all vehicle types is the same at level terrain; and 3) it is unclear whether the “average” truck used in the simulation study for PCE values in the HCM is representative of a typical Nebraska truck. Also, a platoon may form when a truck passes another, resulting in a delay for vehicles that are following, who may wish to be traveling at a faster speed. The objective of this research is to estimate and recommend PCEs for basic freeway segments on I-80 with a high truck percentage in western Nebraska. This research study will examine aspects of the current HCM PCE determination methodology to see if it is representative of Nebraska’s traffic on basic freeway segments. To accomplish these tasks, field data was collected using ITS data collection equipment, including video and radar detectors. This data will be used to: 1) analyze characteristics of platoons on I-80; 2) calibrate a VISSIM 5.4 traffic simulation model that can be used to estimate PCE values in a manner similar to that used to calculate the HCM values; and 3) calculate PCEs using a variety of approaches (e.g., headway-based method and delay-based method). The PCE values under truck restriction conditions are also calculated using these simulation data. The results suggest the PCEs in the HCM 2010 for level freeway segments (1.5) may not be suitable for traffic flow on I-80 in western Nebraska. The PCEs based on the equal-density method (HCM method) using the different speed distributions for trucks and passenger cars with an average of 3.0, and the PCEs based on the delay method with an average of 2.8, are finallyrecommended.