Evaluate the Benefits of Increasing Clear Zone at Higher Speed/Traffic Volume/Crash Locations

The current edition of the AASHTO Roadside Design Guide (RDG) provides guidance to transportation agencies to develop standards and policies for determining the widths of clear zones along roadways based on design speed, traffic volume, roadside slope, and curvature. By providing clear zones, transportation agencies can increase the likelihood that a roadway departure results in a safe recovery and mitigate the severity of crashes that do occur. The development and use of the clear zone concept has dramatically reduced the number of crashes, injuries, and fatalities on urban and rural highways. The RDG provides only a general approximation of the needed clear zone distance. Clear zone recommendations can be extrapolated for design speeds greater than the maximum ranges shown in the 4th edition of the RDG, corresponding to 65 to 70 mph (100 to 110 km/h) and for average daily traffic (ADT) greater than 6,000 vehicles/day or more. However, it is unclear if extrapolated values are optimized for posted speed limits (PSLs) greater than 70 mph (110 km/h) or for roads with ADTs significantly higher than 6,000 vehicle/day. Two of the key factors in assessing risk are design speed and traffic volumes. In some locations in the United States, PSLs have been increased to 80 mph (129 km/h) or more, and there are many segments of highway in which the ADT is greater than 50,000 vehicles/day. Limited data has been collected to evaluate the effectiveness of clear zone recommendations not consistent with the existing ranges shown in the RDG. The width of the clear zone should be based on actual risks. Therefore, there is a need to analyze crash data to determine if revisions to the RDG clear zones are warranted to accommodate increased design speeds, locations with higher crash frequencies, and increased traffic volumes. The guidance developed through this research directly correlates with TCRS’ (define) Strategic Plan and the number one goal in the TCRS mission statement is, “develop, implement, and maintain guidance which will reduce fatal and incapacitating-injury roadway departure crashes.” Results of this research will help guide future editions of MASH, the RDG, and other AASHTO documents. The research objective is to recommend guidelines for clear zone values corresponding to conditions with design speeds, traffic volumes, and crash frequencies in excess of thresholds recommended in the RDG.


  • English


  • Status: Proposed
  • Funding: $450000
  • Contract Numbers:

    Project 07-33

  • Sponsor Organizations:

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001

    American Association of State Highway and Transportation Officials (AASHTO)

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

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Project Managers:

    Crichton-Sumners, Camille

  • Start Date: 20220607
  • Expected Completion Date: 0
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01847416
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 07-33
  • Files: TRB, RIP
  • Created Date: May 23 2022 3:03PM