The National Academies

NCHRP 03-45 [Final]

Speed-Flow Relationships for Basic Freeway Segments

  Project Data
Funds: $350,000
Research Agency: JHK & Associates
Principal Investigator: James M. Schoen
Effective Date: 11/1/1992
Completion Date: 4/30/1995

The final report and draft Chapter 3, "Basic Freeway Segments," were given to the TRB Highway Capacity and Quality of Service Committee for their consideration in revising the Highway Capacity Manual (HCM). The material was incorporated into the 1997 update to the HCM.

The material in Chapter 3, "Basic Freeway Segments," of the 1985 Highway Capacity Manual is based on sparse databases and dated information. Basic freeway flow characteristics and the impact of heavy vehicles and restricted lane/shoulder widths had not been comprehensively studied since the early 1960s. In addition, a number of other factors that may affect speed-flow relationships are either ignored or are not well specified in the 1985 HCM. The 1994 edition of the 1985 HCM raised the freeway capacity to 2200 vehicles per hour per lane based on the results of NCHRP Project 3-33, Capacity and Level of Service Procedures for Multilane Rural and Suburban Highways.

The objective of this research was to revise material on speed-flow relationships in order to update Chapter 3 of the HCM.

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