The National Academies

NCHRP 15-12 [Completed]

Roadway Widths for Low Traffic Volume Roads

  Project Data
Funds: $250,000
Research Agency: Jack E. Leisch & Associates
Principal Investigator: Charles V. Zegeer and Timothy R. Neuman
Effective Date: 5/1/1989
Completion Date: 10/31/1993

The 1984 AASHTO Policy on Geometric Design for Highways and Streets (the Greenbook), established minimum roadway widths for new construction and major reconstruction of roadways based on road classification, traffic volume and design speed. These minimum roadway widths were based on limited research findings augmented with engineering judgment. Recent research indicates, however, that acceptable operational and safety experience can be found on low traffic volume (less than 2000 ADT) roadways of narrower width. Thus it appears that roadway widths less than the minimums specified in the Greenbook might be appropriate for new construction and major reconstruction of low traffic volume roadways.

Presently, about 1.3 million miles of rural roadways or one third of the total rural mileage in the U.S. falls within the low traffic volume category. Several recent studies indicate resources at the state and local level are not sufficient to meet the improvement needs of these low traffic volume roads using the criteria contained in the Greenbook. A reduction in the roadway width requirements could have a major impact on the total program by allowing additional miles of roadway to be constructed or reconstructed with significant operational and safety benefits on a system wide basis.

The objectives of this research were to develop an engineering analysis procedure for determining roadway width for the construction and reconstruction of low volume roadways (less than 2,000 ADT) and, based on this analysis, to develop minimum width of traveled way and shoulder recommendations for consideration by the Geometric Design Task Force of the AASHTO Highway Subcommittee on Design for inclusion in future editions of the Greenbook. This project involved (1) a critical review of all pertinent literature dealing with safety, operations, and geometrics of low volume roads pertaining to roadway width, (2) the collection and analysis of data on traffic volume, vehicle speed, percent trucks, geometrics, functional classification (arterial, collector, and local), level of service, accidents, and associated costs (i.e., traffic accident and roadway construction, renovation, and maintenance), (3) development of an engineering analysis procedure for determining width for roadways with an ADT of less than 2,000, (4) the application of the procedure to develop recommended roadway widths for specific site conditions to determine recommendations for changes to the Greenbook, and (5) assessment of the benefits of implementating the recommended revisions. An extremely large set of low-volume roads was analyzed in the project resulting in a comprehensive and robust analysis of the safety impacts of roadway widths.

Status: The project has been completed, and the results have been published as NCHRP Report 362.

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