Two-lane highway analysis methodology enhancements considering commercial trucks

Two-lane highways are critical components of the highway system, and are continuing to see increased truck traffic along with all other components of the highway network. It is therefore essential to have analysis tools/methods that are sensitive to the unique characteristics of commercial trucks. A significant revision to the Highway Capacity Manual (HCM) two-lane highway analysis methodology was recently completed as part of National Cooperative Highway Research Program (NCHRP) project 17-65. This project made use of a microscopic simulation tool (SwashSim) that performs more detailed truck dynamics modeling than other simulation tools. For example, specific powertrain characteristics (engine, transmission) are used to determine tractive effort, and roadway and physical vehicle characteristics (e.g., weight, frontal area, coefficient of drag) are used to determine resistance forces. These variables are used to calculate maximum acceleration and velocity (values that, illogically, are often user inputs in simulation programs). Overall, the resulting revised methodology from this work better accounts for the unique operating characteristics of commercial trucks on traffic stream operational performance. However, there are still several key areas in the new methodology where further investigation is warranted: (1) Passing lane performance for various diverge/merge rules faster/slower vehicles. (2) Many passing lane configurations require “slower drivers keep right”, which usually entails the slower vehicles move over to the added lane and remerge to the regular lane downstream before the added lane ends. Since commercial trucks are usually slower vehicles, their merging from the added lane to the regular lane at the lane drop area can cause disruptive turbulence at the merge point when traffic flows moderately high. Some alternative passing lane designs are starting to appear, such as slower vehicles moving right at the start of the passing lane segment, but faster vehicles having to merge at the end of the passing lane, and ‘2+1’ type of configurations where the fasters vehicles need to change lanes at both the start and end of the passing lane segment. The relative impacts to the traffic stream performance due to these different designs needs to be better understood, particularly for traffic streams with non-trivial percentages of commercial trucks. (3) Guidance for climbing lane design (length and return to level grade conditions) (4) The merging behavior of trucks at the end of a passing lane segment can be even more problematic on upgrades (i.e., a climbing lane), as the speed differential between the trucks and passenger cars can be quite significant. The AASHTO Green Book recommends that a passing lane on a grade be continued onto a relatively level segment of roadway until the truck speeds are at a minimum of 40 mi/h and within 10 mi/h of the passenger car speed. Some quantitative guidance on expected lengths of passing lane needed to achieve smooth reintegration of trucks to the regular lane, based on overall flow rate, grade %, and truck %, is needed. (5) Effective length of passing lane (6) In the NCHRP 17-65 project, quantification of the effective length of a passing lane (i.e., distance downstream of the passing lane for which the improvements to the performance measures last) was only determined for level terrain. On non-level terrain, commercial trucks can have a significant impact on this distance. Additional quantitative guidance is needed for the effective length of passing lanes on non-level terrain, when trucks are present in the traffic stream. (7) Capacity on non-passing lane upgrade segments when trucks are present in traffic stream (8) Field data collected as part of NCHRP 17-65 did not yield enough very high flow rate conditions to make meaningful insights into the concept of capacity. Capacity was investigated, through simulation, for passing lane segments (which was constrained by the downstream merging operations). However, capacity, for which trucks can have a significant influence, was not examined for non-passing lane segments. This issue needs further examination, particularly for non-level terrain. This project aims to improve the state-of-the-art for accounting for the impact of trucks on two-lane highway operations. This will be accomplished by building on the work that was done for NCHRP Project 17-65. The issues examined in this project are ones which are very difficult and/or very expensive to study in the field. Thus, the SwashSim simulation tool will be utilized exclusively in this project. SwashSim has the ability to model a wide range of two-lane highway configurations and operational scenarios. Because of its detailed vehicle dynamics modeling approach, SwashSim is also well-suited to modeling situations that are sensitive to the impacts of commercial vehicles. Furthermore, through the work of the NCHRP Project 17-65, SwashSim went through an extensive calibration effort with field data.

  • Supplemental Notes:
    • Contract to a Performing Organization has not yet been awarded.


  • Status: Active
  • Funding: $85000
  • Contract Numbers:


  • Sponsor Organizations:

    United States Department of Transportation - FHWA - LTAP

    1200 New Jersey Avenue, SE
    Washington, DC    20590

    Office of the Assistant Secretary for Research and Technology

    University Transportation Centers Program
    Department of Transportation
    Washington, DC  United States  20590
  • Managing Organizations:

    Freight Mobility Research Institute

    Florida Atlantic University
    Boca Raton, FL  United States  33431
  • Principal Investigators:

    Washburn, Scott

  • Start Date: 20180701
  • Expected Completion Date: 0
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01682649
  • Record Type: Research project
  • Source Agency: Freight Mobility Research Institute
  • Contract Numbers: 69A3551747120
  • Files: UTC, RiP
  • Created Date: Oct 3 2018 11:52AM