The National Academies

NCHRP 07-33 [Pending]

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

  Project Data
Funds: $450,000
Contract Time: 30 months
Staff Responsibility: Camille Crichton-Sumners


The current edition of the AASHTO Roadside Design Guide (RDG) provides guidance on developing standards and policies for determining the widths of clear zones along roadways based on design speed, traffic volume, roadside slope, and road curvature. By providing clear zones, transportation agencies can increase the likelihood that a roadway departure results in a safe recovery and mitigates the severity of crashes that occur. 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 RDG, fourth edition, 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 speeds greater than 70 mph (110 km/h) or for roads with ADTs significantly higher than 6,000 vehicle/day at all speeds.

Two of the key factors in assessing risk are design speed and traffic volumes. In some locations in the United States, posted speed limits (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. Clear zone values should be based on actual risks. Therefore, there is a need to analyze crash data and other factors (i.e., human and environmental factors) to determine if revisions to the RDG clear zones are warranted to accommodate increased design speeds, increased traffic volumes, and other factors that may contribute to higher crash frequencies. Results of this research will help guide future editions of the AASHTO RDG, Manual for Assessing Safety Hardware (MASH), and other traffic safety documents.


The research objectives are to (1) identify factors influencing clear zone values and (2) recommend clear zone values corresponding with design speeds and traffic volumes in excess of thresholds recommended in the AASHTO RDG.


Task 1. Perform a review of scholarly literature related to the development of clear zones and summarize the findings in a Task 1 Report that includes factors considered in determining clear zone values.

Task 2. Identify the state of practice related to how state departments of transportation (DOTs) determine clear zone values beyond the guidance within the AASHTO RDG.

Task 3. Develop a methodology for determining clear zone values and thresholds for high speed and high volume roadways.

Task 4. Prepare an Interim Report No.1 that documents the work completed in Tasks 1 through 3. Include a detailed work plan for the work anticipated in Phase II.

Task 5. Execute the methodology according to the approved Interim Report No. 1.

Task 6. Document the method for determining clear zone values and prepare draft language for consideration by AASHTO to incorporate the research results in a future update of the AASHTO RDG.

Task 7. Identify steps DOTs can take to quantify implementation costs for clear zones.

Task 8. Prepare final deliverables.


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