Laurence R. Rilett
Elizabeth G. Jones
Date of this Version
Doctoral Dissertation, University of Nebraska-Lincoln, 2018
The Highway Capacity Manual (HCM) was developed to provide capacity and level of service analyses for roadway facilities. Trucks may adversely affect the quality of traffic flow on a roadway. In HCM, the passenger car equivalent (PCE) of a truck, which represents the number of passenger cars that have an equivalent effect on traffic flow, is used to account for the impacts of trucks. However, in the past ten years rural freeways in the western rural U.S. have experienced conditions that lie outside the standard HCM conditions. Also, the current HCM truck PCEs may not be appropriate for the western rural U.S. This is because, the interstates in the western rural U.S. consistently experience truck percentages in an excess of 25 percent, but the highest truck percentage published in current HCM is 25 percent. Additionally, there are large free-flow speed differences between heavy trucks and passenger cars in western rural U.S., however, the current HCM estimates the PCEs under the assumption that trucks maintain the same speed as passenger cars on level terrain. Compounding the above two issues, trucks passing other trucks at low speed differentials may cause moving bottlenecks. This dissertation proposed a definition, developed identification methods for the moving bottlenecks on four-lane freeway segments, and developed metrics for measuring their effects. Then, this dissertation calculated PCEs under western rural U.S. traffic flow conditions with localized congestion caused by moving bottlenecks, by equal-density and equal-capacity method. Finally, this dissertation explored the impacts of changes in speed limits, truck passing restriction and data aggregation interval on PCEs. The results demonstrate moving bottlenecks have an adverse effect on vehicles on the freeway. It was found that the PCE values in the HCM 2010 and HCM 2016 underestimate the effect of heavy trucks on level terrain freeways that experience high truck percentage, and where different vehicle types have large differences in average free-flow speeds. The results also show that speed limits, percentage of truck passing restriction, and data aggregation interval significantly affect the PCEs. The results will be helpful in understanding how trucks affect passenger cars in moving bottlenecks.
Advisor: Laurence R. Rilett (Chair), Elizabeth G. Jones (Co-chair)