Analysis of Oversaturated Traffic Flow Conditions on Freeway Facilities

The 2000 edition of the Highway Capacity Manual (HCM) was the first to handle the analysis of oversaturated freeway segments over multiple time intervals. Although this was a significant improvement, there are some major areas and gaps that need to be addressed in the current procedures. The speed-flow curves that were developed for basic freeway sections were based on field data collected in the 1990s. While a significant amount of data representing oversaturated flow was collected, definition of a relationship in this regime was not possible due to the high variability in the data. Therefore, the speed-flow curves in the HCM only include the under-saturated flow regime. This was a significant issue during the development of the freeway facilities procedures. To address the issue, a simple linear flow-density relationship was assumed to model the associated speed-flow relationship representing the oversaturated regime. This assumed relationship has not been extensively validated with field data. Initial results were promising (Hall et al., TRR 1710, 2000), but far from comprehensive. A better understanding of the speed-flow-density relationship in the oversaturated regime will result in greater accuracy in estimating freeway operations under congested flow, in particular, how far back will queues extend upstream of bottlenecks or incidents. Since many freeways in metropolitan areas now operate this oversaturated regime for part of the day, a better understanding of these operations is critical for accurate analysis. Additional research is also needed on the apparent loss of capacity once oversaturated conditions occur. Previous research has shown that the queue discharge rate appears to be 5 to 10 percent less than the capacity than can be achieved in under-saturated flow. The HCM does not take this reduction into account. The loss of only 5 percent of the capacity at a bottleneck can have a large impact on queue formation and dissipation, and facility throughput. The HCM procedure is currently limited to a single bottleneck or multiple, non-interacting bottlenecks. A better understanding of traffic flow within multiple, interacting bottlenecks is needed so that the HCM model can be modified to provide a more realistic estimate of oversaturated flow. Multiple, interacting bottlenecks are common, so incorporation of these into the existing model would greatly increase the usefulness of the HCM procedures. The interface of the freeway facility operation with the surface street should also be improved. Most notably, the current HCM procedure does not account for the effect of capacity restrictions at off-ramps, which may cause queuing and back-ups onto the freeway mainline (e.g. due to a downstream signal). Furthermore, queues that may occur at the on-ramps are modeled “vertically,” and therefore do not realistically represent the effect of such queues on surface street operations. The objective of the research is to improve and enhance the HCM 2010 methodology of the operational analysis of freeway facilities in oversaturated conditions. The research should take related efforts into account, particularly the incorporation of travel time reliability into the HCM.  


  • English


  • Status: Proposed
  • Contract Numbers:

    Project 03-96A

  • Sponsor Organizations:

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Project Managers:

    Derr, B

  • Start Date: 20171219
  • Expected Completion Date: 0
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01654287
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 03-96A
  • Files: TRB, RiP
  • Created Date: Dec 18 2017 3:03PM