The National Academies

NCHRP 15-26 [Final]

Passing Sight Distance Criteria

  Project Data
Funds: $300,000
Research Agency: Midwest Research Institute
Principal Investigator: Douglas W. Harwood
Effective Date: 6/1/2004
Completion Date: 9/30/2007


The design criteria for minimum passing sight distance (PSD) for two-lane highways in AASHTO's 2001 A Policy on Geometric Design of Highways and Streets (the "Green Book") have remained virtually unchanged since they were incorporated into the 1954 edition of the policy. The 1954 policy used criteria based on a summary report of extensive field observations of passing maneuvers made during 1938 to 1941 by Prisk and published in Proceedings HRB Volume 21. Surveys conducted in 1971 and 1978 found that AASHTO values for PSD were conservative, except at passing vehicle speeds above 65 mph, and these values continued to be used in the 1984, 1990, 1994, and 2001 editions. Findings from other studies have been presented in various forums discussing PSD subsequent to 2001, but the PSD values in the Green Book remain unchanged. The vehicle fleet, operating conditions, and characteristics of the driver have changed considerably over the past 50 years, but the current PSD criteria do not take these changes into account. Furthermore, the operational criteria in the FHWA Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD) and the design criteria in the Green Book result in different minimum passing sight distance values for similar conditions, and there has been considerable discussion on whether these differences should be rationalized or reconciled.

This research was needed to provide passing sight distances consistent with today's conditions and to resolve inconsistencies between existing criteria. The results will be valuable for highway design and operations nationwide and will be considered for incorporation in future editions of both the Green Book and the MUTCD.


The objectives of this study were to evaluate the design and operational criteria for determining minimum passing sight distance and to modify or develop new PSD criteria for the Green Book and the MUTCD.

Accomplishment of the project objectives required the following tasks.


PHASE I (1.) Review and summarize the principles, data, assumptions, and calculations behind the criteria used to determine minimum PSD from a design standpoint in the current AASHTO Green Book and from an operational standpoint in the MUTCD. (2.) Analyze, describe, and critique pertinent domestic and international research and practices, on the basis of applicability, conclusiveness of findings, and usefulness for the determination of PSD. Include completed research and research currently underway. (3.) Identify the full range of factors that potentially contribute to PSD requirements. It is expected that these would include, but not be limited to, performance characteristics and configuration of the passenger and commercial vehicle fleet, driver characteristics and behavior, operating conditions (e.g., traffic volumes and speed), terrain, roadside features, and roadway geometry. (4.) Based on the information developed in Tasks 1, 2, and 3, analyze and critique the current criteria for determining PSD. Identify limitations and develop recommendations for acceptance or modification of these criteria. If additional research is needed to prepare recommendations, prepare a research plan for Phase II which will provide for the collection and analysis of additional information and the development of new or modified criteria to determine PSD. (5.) Submit an interim report, within 7 months, to document Tasks 1 through 4 for review by the NCHRP. The contractor will be expected to meet with the NCHRP panel approximately 1 month later. A decision whether or not to proceed with any or all of proposed Phase II activity will be made by the NCHRP. The contractor shall not begin work on Phase II without NCHRP approval.

PHASE II (6.) Perform the research outlined in the Task 4 plan as approved by the NCHRP panel. (7.) Prepare new or modified PSD criteria in a format suitable for inclusion in the Green Book and MUTCD. (8.) Submit a final report that documents the entire research effort. In addition, provide a companion executive summary that outlines the research results and recommendations. The deliverables will also include a PowerPoint presentation which summarizes the background, methodology, and results of the study. The suggested modifications to the Green Book and MUTCD shall be included as separate appendices. (9.) Prepare and deliver presentations on the project results to both the AASHTO Task Force on Geometric Design and to the National Committee on Uniform Traffic Control Devices.

Status: The final report has been published as NCHRP Report 605. 

Product Availability: The final report can be purchased or downloaded from the TRB website.  A PowerPoint slideshow on the project is also available.

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