Sag Vertical Curve Design Criteria for Headlight Sight Distance

Headlamp sight distance is one of four design criteria for sag vertical curves and is the most often used of the four criteria. The current criterion bases the length of a sag curve on the distance illuminated by a headlamp beam that diverges at 1 degree above the horizontal. This criterion was developed in the late 1930s and has remained unchanged since except for a decrease in the headlamp height (from 2.5 to 2.0 ft in the 1965 Blue Book). At the time the criterion was developed, the sealed beam headlamp was established as the standard headlamp system for U.S. vehicles and the sealed beam headlamp continued to be the standard headlamp into the mid-1980s. However, starting in the mid-1980s, vehicle manufacturers began introducing changes in headlamp design with varying headlamp performance. A more detailed research study is needed to update the sag curve sight distance criteria so that it reflects the performance of the modern vehicle fleet. The design criteria for sag curves have not been investigated in detail in over 60 years. There have been significant changes in vehicle design/performance and in driver perception during the ensuing years. It is appropriate to evaluate the current design criteria to determine whether the basis for design is still valid and whether improvements can be realized through revised design criteria. Revised criteria could improve safety, operations, and/or reduce construction costs. Based on the exploratory research conducted to date, it is expected that this research will produce recommendations for changes in the design criteria for sag curves. The changes could be as simple as a reduction in the 1.0 degree α angle currently used to a more extensive change such as a new criteria based on a different concept for sag curves. The objectives of this research are to (1) develop updated criteria and guidance for the design of sag vertical curves to provide the needed headlight sight distance with today's vehicles and (2) assess the practical implications of the criteria and guidance.


  • English


  • Status: Completed
  • Contract Numbers:

    Project 15-41

  • Sponsor Organizations:

    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

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Derr, B

  • Performing Organizations:

    Virginia Polytechnic Institute and State University, Blacksburg

    208 Patton Hall
    Blacksburg, VA  United States  24061
  • Principal Investigators:

    Gibbons, Ronald

  • Start Date: 20100524
  • Expected Completion Date: 0
  • Actual Completion Date: 20121124
  • Source Data: RiP Project 22362

Subject/Index Terms

Filing Info

  • Accession Number: 01464191
  • Record Type: Research project
  • Source Agency: National Cooperative Highway Research Program
  • Contract Numbers: Project 15-41
  • Files: TRB, RiP, USDOT
  • Created Date: Jan 3 2013 2:38PM