Horizontal Sightline Offset Criteria for Freeways, Ramps, and Other Similar Roadways

When horizontal curves are bound by barrier walls or other potential sightline impediments (i.e., retaining walls, parapet walls, abutments, piers, columns, and sound barrier), especially in high-speed roadways and system ramp connections, designers compensate for the impact to driver sight distance in various ways including lowering design speed, increasing shoulder width, providing additional signage, or other mitigation strategies. There are drawbacks to each of these mitigation strategies. Lowering the design speed can result in decreased superelevation without any other visual cues for the driver to lower their speed. Another adverse result can lead to extra wide shoulders (ranging from 16 feet to 20+ feet) along the inside of the horizontal curve when the barrier/‌visual impediment is located on the same side. Providing extra wide shoulder increases costs (especially if on structure), may increase right of way (ROW) needs, and may encourage erratic behavior from drivers who believe the additional area was intended for parking or as an additional driving lane (this can lead to operational and safety implications). In addition, these concerns may prevent agencies from installing high occupancy vehicle (HOV) lanes to manage capacity problems. Because of these conflicting factors, many agencies have used the design exception processes to address the tradeoffs for sight distance in such situations. This research would examine these situations to determine whether criteria for horizontal sightline offset could be developed to better address these conflicting factors. The research should evaluate this condition and determine what criteria will provide an acceptable design. Research should examine existing research, current design criteria, performance, and safety of existing locations with such geometrics; identify mitigation techniques; and determine whether the design criteria should be revised. The researchers should take into consideration the impact of the vertical alignment in addition to the horizontal offset to the wall or impediment. The researchers should also examine the geometrics under various traffic conditions. Dense traffic may be another impacting factor in determining the highway safety office (HSO) criteria. In the case of barrier wall and parapets, glare screens may also be a desirable design element. The objective of this research is to provide horizontal offset design criteria for curved alignments adjacent to barrier and similar types of impediments that may impact the driver's line of sight. The research would be helpful to the transportation design professional and facility owners. The findings may lead to modification of the design criteria and could reduce costs, reduce ROW needs, and eliminate some future design exceptions. The results of the project should be developed in a manner that allows it to be directly used within future updates to the American Association for State Highway and Transportation Officials (AASHTO) Green Book.


  • English


  • Status: Proposed
  • 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

    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

  • Start Date: 20141111
  • Expected Completion Date: 0
  • Actual Completion Date: 0
  • 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