The National Academies

NCHRP 03-55(3) [Completed]

Capacity and Quality of Service for Two-Lane Highways

  Project Data
Funds: $600,000 (NCHRP--$300,000; FHWA--$300,000)
Research Agency: Midwest Research Institute
Principal Investigator: Douglas W. Harwood
Effective Date: 9/15/1995
Completion Date: 11/30/1999

The 2000 edition of the Highway Capacity Manual (HCM 2000) is available at the TRB Bookstore. This edition includes new material on analyzing two-lane highways that was developed in NCHRP Project 3-55(3). Some of the significant changes from the previous edition are the following:
  • There are now two classes of two-lane highways. On Class I highways, efficient mobility is paramount, and level of service is defined in terms of both percent-time-spent-following and average travel speed. On Class II highways, mobility is less critical, and level of service is defined only in terms of percent-time-spent following. The HCM user decides which highway class is appropriate using his or her assessment of driver expectations.
  • Each direction of travel can be analyzed separately in level and rolling terrain and for specific upgrades and downgrades.
  • Passing and climbing lanes can be analyzed.

During the course of the project, several improvements were made to TWOPAS, a computerized simulation model for analyzing the operation of two-lane rural highways. The revised model was delivered to the FHWA for further use. Work is now underway to develop a stand-alone version of TWOPAS and to incorporate it into the Interactive Highway Safety Design Model (IHSDM). These efforts are expected to be completed by the end of 2001.

The agency final report describing the research conducted is available below in portable document format (PDF). (A free copy of the Adobe Acrobat Reader is available at https://www.adobe.com.)

Final Report (2,313 kbytes)
Appendices (315 kbytes)

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