Horizontal Sightline Offset Design Criteria, Exceptions, and Mitigation Strategies

When the sight lines along horizontal curves are impaired, designers are challenged to find acceptable solutions. Designers compensate for the limitations on driver sight distance in various ways, including: accepting deficient sightlines, lowering design speed, increasing shoulder width, or providing additional signage. There are advantages and disadvantages to the trade-offs being made; therefore, many agencies have used the design exception process to address the trade-offs for sight distance in such situations. Research is needed to evaluate these situations and determine what criteria or mitigation will provide acceptable solutions when impaired horizontal sightline offsets are encountered. The objectives of this research are to: (1) evaluate the safety and operational performance as well as the trade-offs and risks associated with state-of-the-practice mitigation treatments when horizontal sightline offset criteria and guidance are not met; and (2) recommend updates to AASHTO’s Policy on Geometric Design of Highways and Streets (Green Book) for horizontal sightline offset design criteria and guidance on curved roadway alignments adjacent to barriers or other types of impediments that may impact the driver’s line of sight. The research should address a broad range of issues related to horizontal sightline offset design criteria and guidance for curved roadway alignments and may include, but not be limited to, the following: (1) Review of research, practice, and innovative approaches in the United States and other countries that complement the research objective, including substantive approaches to assessing safety for different types of roadways. (2) Identification of factors (e.g., human factors, vehicle characteristics, vertical alignment, construction costs, and other built or natural environmental conditions) that influence horizontal sightline offset design and performance. (3) In situations where horizontal sightline offset design criteria could not be met, what trade-offs and risks were considered in choosing the final design elements (e.g., shoulder width, barrier height, design speed)? What elements are most commonly considered to best resolve deficiencies? What mitigation strategies were implemented and how were they chosen? How did the facilities perform after mitigation? (4) In future situations when horizontal sightline offset criteria cannot be met, develop guidance on what trade-offs and risks should be considered, what elements are most likely to best resolve deficiencies, and what mitigation strategies should be evaluated based on risks and trade-offs. (5) Proposed new text for the next edition of AASHTO’s Policy on Geometric Design of Highways and Streets (Green Book).


  • English


  • Status: Completed
  • Funding: $400000
  • Contract Numbers:

    Project 15-59

  • Sponsor Organizations:

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway & Transportation Officials (AASHTO)

    444 North Capitol Street, NW, Suite 225
    Washington, DC  United States  20001

    National Cooperative Highway Research Program

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

    Rogers, William

  • Performing Organizations:


    425 Volker Boulevard
    Kansas City, MO  United States  64110-2241
  • Principal Investigators:

    Potts, Ingrid

  • Start Date: 20150810
  • Expected Completion Date: 20180831
  • Actual Completion Date: 20180831
  • Source Data: RiP Project 37547

Subject/Index Terms

Filing Info

  • Accession Number: 01543355
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 15-59
  • Files: TRB, RiP
  • Created Date: Nov 13 2014 1:01AM