The National Academies

NCHRP 15-41 [Final]

Sag Vertical Curve Design Criteria for Headlight Sight Distance

  Project Data
Funds: $250,000
Research Agency: Virginia Polytechnic Institute
Principal Investigator: Ronald Gibbons
Effective Date: 5/24/2010
Completion Date: 11/24/2012

The objectives of this research were 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.
The AASHTO Policy on Geometric Design of Highways and Streets (Green Book) lists four criteria for establishing the length of sag vertical curves. Of these, headlight sight distance (the length of highway illuminated by the vehicle’s headlamps) is the one that most often governs the design (the others are passenger comfort, drainage control, and general appearance). The current headlight sight distance criterion was developed in the late 1930s and has remained unchanged except for a 1965 decrease in the headlamp height (from 2.5 to 2.0 ft). The sealed beam headlamp was the standard system for U.S. vehicles from the 1930s into the mid-1980s. At that time, vehicle manufacturers began changing headlamp designs, resulting in changes in photometric performance.  Of particular importance to headlight sight distance is the reduced amount of light projected above the longitudinal axis of the vehicle. Thus, research is needed to evaluate the current design criteria to determine whether the basis for design is still valid and whether revised design criteria are worthwhile.  
Task 1.  Review domestic and international (particularly European) literature on driver and vehicle headlight characteristics relevant to the design of sag vertical curves, alternative design approaches for sag vertical curves, and non-geometric treatments for sag vertical curves. Determine how many state and international DOTs follow the AASHTO Green Book approach to designing sag vertical curves and document alternative design approaches in use.

Task 2.  Based largely on the literature, characterize the photometric performance of the headlamps of today’s vehicle fleet and the illumination needs of the driving population related to the driver’s ability to see along a sag vertical curve. 

Task 3.  Identify promising modifications to the existing AASHTO Green Book approach for the design of sag vertical curves and alternative design approaches. 

Task 4.  Within 3 months of the contract start, submit an interim report documenting the results of Tasks 1 through 3 and presenting an updated work plan for the remaining tasks including specific evaluation protocols for Task 5. Meet with the NCHRP Project Panel to review the interim report before proceeding with the remaining tasks.

Task 5.  Evaluate the effectiveness of the current and alternative design approaches in providing headlight sight distance for sag vertical curves. Compare their likely impacts on safety-related driver performance. Assess the approaches’ compatibility with the other three design criteria for sag vertical curves mentioned in the AASHTO Green Book.

Task 6.  Assess the practical implications of the alternative design approaches on construction costs, environmental impacts, right-of-way requirements, and other non-safety factors.

Task 7.  Submit a final report documenting the entire research effort. The report should include an appendix showing suggested text changes to the section of the AASHTO Green Book titled “Sag Vertical Curves.” The report should document the headlamp configurations used in the assessment and summarize information on non-geometric treatments for sag vertical curves identified in the course of the research. 

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