Applicability of the 85th Percentile for Setting Speed Limits on Freeways, Expressways, and Rural Highways

The applicability of the 85th percentile speed for determining posted speed limits is a complex and controversial issue of concern to state departments of transportation (DOTs). The 85th percentile is the speed at or below which 85 percent of free-flowing traffic travels. It is sometimes used to provide an indication of the free-flow operating speed on the roadway for determining traffic control device applications. It has been used for decades as the basis for setting speed limits. In recent years, calls have been made to eliminate the 85th percentile speed as the basis for setting speed limits, especially in urban areas. Proponents of the 85th percentile speed argue that (1) it is a good measure of the speed at which drivers feel safe and comfortable on a given road; (2) it is a relevant data point that can indicate whether other modifications or speed management strategies might be needed to achieve compliance or some level of a self-enforcing road design; (3) and setting speed limits at or near the 85th percentile speed can reduce speed variance and improve safety. Conversely, critics argue that (1) the 85th percentile speed is not a reliable measure of safe driving; (2) it can be influenced by many factors, such as traffic volume, road conditions, and drivers’ perception of law enforcement activity; and (3) setting speed limits at or near the 85th percentile speed can encourage drivers to travel at higher speeds. Therefore, critics argue that the 85th percentile speed should be removed as a factor and that the Safe System approach should be required instead. Recent research and reasoning for moving away from the 85th percentile speed on urban arterials and residential streets do not address speed limits on freeways, expressways, and rural highways. Therefore, research is needed to examine the applicability of the 85th percentile speed for setting speed limits on freeways, expressways, or rural highways. OBJECTIVES: The objectives of this project are to (1) examine the applicability of the 85th percentile speed as a factor in setting speed limits on freeways, expressways, and rural highways and (2) prepare a guide for state DOTs and other agencies with the authority to set speed limits on freeways, expressways, and rural highways that includes, at a minimum, implementation considerations, an application framework, and outreach materials for communication with policymakers. 

• Record URL:
  • http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=5681
• Supplemental Notes:
  • Contract to a Performing Organization has not yet been awarded.

Language

• English

Project

• Funding: $500000
• Contract Numbers:

  Project 17-133

• Sponsor Organizations:

  National Cooperative Highway Research Program

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

  Federal Highway Administration

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

  American Association of State Highway and Transportation Officials (AASHTO)

  444 North Capitol Street, NW
  Washington, DC  United States  20001
• Project Managers:

  Retting, Richard

• Start Date: 20241216
• Expected Completion Date: 0
• Actual Completion Date: 0

Subject/Index Terms

• TRT Terms: Benefit cost analysis; Crash rates; Driving behavior; Freeways; Highway safety; Rural highways; Speed limits; State departments of transportation; Traffic engineering
• Subject Areas: Highways; Operations and Traffic Management; Safety and Human Factors;

Filing Info

• Accession Number: 01919344
• Record Type: Research project
• Source Agency: Transportation Research Board
• Contract Numbers: Project 17-133
• Files: TRB, RIP
• Created Date: May 22 2024 11:45AM