Highway Capacity Manual Methodologies for Analyzing Freeway Merging and Diverging Bottlenecks Considering Different Geometrics Characteristics and Mitigation Strategies

Freeway congestion typically generates at freeway merge and diverge segments, which have the potential to serve as bottlenecks. To alleviate or mitigate the impacts of congestion at these segments, a number of active management operational strategies such as ramp metering, hard shoulder running, etc. have been implemented. The current freeway merge and diverge methodologies in Chapter 14 of the 6th edition of the Highway Capacity Manual (HCM) were developed over 25 years ago using limited field-collected data. In addition to limited data, the methodology does not conform to the fundamental relationship of traffic flow, namely that flow is the product of speed and density. Also, contrary to literature findings, the methodology overestimates the capacity values at both merge and diverge segments. In addition, the HCM does not offer any methodology for lane drops or additions, which often occur in the vicinity of freeway merge/diverge segments. The objectives of this research are to (1) evaluate the current HCM freeway merge and diverge methodologies and propose methodology improvements or changes, (2) consider the effects of commonly implemented freeway management tools and strategies on capacity and quality of service, and (3) evaluate different geometric configurations that have not been adequately described in the HCM, such as lane drops/additions or long acceleration/deceleration lanes for multilane ramps, and recommend modifications to the proposed methodologies accordingly. The research will evaluate the use of emerging data sources for model development and provide guidance on incorporating such datasets for validation of analysis results. The new methodologies should be developed with close consideration to the HCM weaving and basic segment methodologies to ensure compatibility. This research will produce new analytical methodologies and tools for improving the Highway Capacity Manual merge and diverge segments’ existing procedures. The proposed research will address several limitations of the current methodologies, which include different geometric configurations and implementation of active traffic management operational strategies. Failure to fund this problem statement will result in cost-prohibitive use of microsimulation.


  • English


  • Status: Proposed
  • Funding: $500000
  • Contract Numbers:

    Project 07-26

  • Sponsor Organizations:

    National Cooperative Highway Research Program

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

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Dekelbab, Waseem

  • Start Date: 20180813
  • Expected Completion Date: 0
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01672239
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 07-26
  • Files: TRB, RiP
  • Created Date: Jun 18 2018 3:02PM