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FHWA Highway Safety Programs


Following are some of the key points included in this Informational Guide:

  • As speed increases, crash severity increases. The relationship between the probability of a crash and speed is less clear.
  • The ability to accurately predict speeds on all road and street types does not exist nor is there reliable guidance available on how to attain specific operating speed characteristics and speed relationships in geometric design.
  • Speed limits are not always known during the design process.
  • The design process does not include a speed-prediction component.
  • Inferred design speeds are sometimes substantially higher than the designated design speed.
  • After a facility is open to traffic, the 85th percentile speed may be higher than the anticipated and designated design speeds, particularly on low and moderate speed roads.
  • The posted speed may change after the road has been designed, built and open to traffic.
  • The design speed is NOT the maximum safe speed.
  • Reducing the posted speed without other changes is likely to result in a small reduction in operating speeds (i.e., significantly less than the amount of the posted speed change).
  • Posted speeds should almost always be within 5 mph of the 85th percentile speed.
  • The posted speed should not be established based on an isolated restrictive feature (e.g., sharp curve or limited sight distance) within a segment. The use of an advisory speed should be considered at these locations.
  • Speed management is a comprehensive approach to determining and attaining appropriate speeds through a coordinated effort of driver education, enforcement and engineering efforts.
  • Additional information is available from:
    • The Green Book, MUTCD, and state driver’s manuals.
    • The references listed in this Informational Guide.
    • FHWA Speed Management web page (
    • Local Technical Assistance Program (LTAP) and technology transfer centers.