How Should Speed Limits Be Set? At the 85th percentile, of course! 

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Martin R. Parker. "Comparison of Speed Zoning Procedures and their Effectiveness: Final Report." Michigan Department of Transportation - Traffic and Safety Division, 1992 September, pp. vii, 31:
"The available evidence suggests that posting limits in the region of the 85th percentile minimizes accident involvements and provides acceptable driver compliance. There is no information that suggests including other factors in setting speed limits would provide additional safety or compliance benefits."

"Contrary to widespread popular belief, the results indicate that raising speed limits to the 85th percentile does not increase accidents. Also, arbitrarily lowering speed limits more than 5 mi/h below the 85th percentile speed does not reduce accidents."

Ian B. Greenwood. Traffic and Safety Policy Sub-Committee on Speed Cameras. Impact of Speed Cameras. April 1992, p. 2.
"Theoretically, speed limits should be set at close to the 85th percentile speed."

Martin R. Parker. "Comparison of Speed Zoning Procedures and their Effectiveness: Final Report." Michigan Department of Transportation - Traffic and Safety Division, 1992 September, p. vii.
"The 85th percentile speed is the primary factor states use in setting speed limits."

P. E. Spitz. "Speed vs Speed Limits in California Cities." ITE Journal, 54.5 (1984 May), p. 42.
"The basic element in the determination of the appropriate limit is the measured 85th percentile speed of the highway users."

P. N. Seneviratne and M. N. Islam. "Speed Estimates for Roadway Design and Traffic Control." Transportation Research Record, 1375 (1992), p. 37.
"[Estimated 85th percentile speed] is currently the critical (decision) speed in Quebec for posting limits."

Martin R. Parker. "Comparison of Speed Zoning Procedures and their Effectiveness: Final Report." Michigan Department of Transportation - Traffic and Safety Division, 1992 September, p. 3.
"The 1990 American Association of State Highway and Transportation Officials' (AASHTO) A Policy on Geometric Design of Highways and Streets does not provide a specific recommendation, but suggests that the posted maximum speed is about the 85th percentile speed."

Davey L. Warren. "Speed Zoning and Control." Chapter in Synthesis of Safety Research Related to Traffic Control and Roadway Elements, Volume 2, (1982 December), p. 17-7.
"The most widely accepted method of setting speed limits is the 85th percentile speed."

Institute of Transportation Engineers. "Speed Zone Guidelines: A Proposed Recommended Practice." Washington, DC, 1993.
"It is ... recommended that the speed limit ... be set at the nearest 5 mph increment to the 85th percentile speed or the upper limit of the 10 mph pace."

FHWA Study Tour for Speed Management and Enforcement Technology, 1995 December, p. 48.
"In the vast majority of cases, the program [NLIMITS and VLIMITS, used to set speed limits in Victoria, Australia] recommends a speed limit within 5 km/h of the 85th percentile speed."

Martin R. Parker. "Effects of Raising and Lowering Speed Limits: Final Report." U.S. Department of Transportation, Federal Highway Administration, 1992 October, p. 54.
"Accidents ... where speed limits were lowered increased by 5.4 percent."

"Accidents at the ... sites where speed limits were raised decreased by 5.4 percent."

"Lowering speed limits more than 5 mi/h (8 km/h) below the 85th percentile speed of traffic did not reduce accidents."

Texas State Department of Highways and Public Transportation, 1990. "Speed Zoning on Texas Highways"
"Laws are written to single out the unreasonable behavior of a minority of the population... The Texas speed law is written so that the reasonable actions of the majority are considered legal."

Texas speed zoning is based upon the 85th percentile. Safe, simple, democratic.

Rev: 1998.09.29