Speed Limit Fact Sheet ( PDF )
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Statutory Requirements and Standards for Establishing Speed Limits

Speed limits should represent the maximum safe and reasonable speed on a highway during good traffic and roadway conditions. Traffic engineering studies have found that the best way to ascertain the appropriate speed limit is to survey the speeds of free-flowing traffic. The speed at which 85% of the vehicles are traveling at, or below, has generally been determined to be a limit which minimizes accident risk and maximizes motorist compliance. It blends an optimum combination of efficiency, consensus, enforceability, and safety.

Current statutorily assigned speed limits are characteristically inflexible and based on general approximations or political considerations. The result is that speed limits have become largely irrelevant as a source of guidance to motorists and impractical as a threshold for enforcement purposes.

The proposed speed zoning statute will overcome the failings and limitations evidenced by current state laws and practices. It provides a scientific basis to establish a uniform and flexible system of speed zoning that will result in safe, reasonable and relevant speed limits. It allows for local roadway and traffic conditions and accommodates changing trends in vehicle speeds. It is further based on the knowledge that the vast majority of motorists are reasonable and responsible people who will comply with properly established speed limits.


The following definitions are provided to aid in the understanding of this model legislation. They may or may not coincide with terms and definitions found in related state statutes.

Statutory Speed Limits:
Numerical speed limits that apply to various classes or categories of roads (e.g. rural expressways, residential streets, primary arterials, etc.) in the absence of posted speed limits.

Posted Speed Limits:
Numerical speed limits noted on signs or other information displays and placed along the roadway corridor to which they apply.

Prima Facie Speed Limits:
Numerical speed limits (statutory and/or posted) that, if exceeded, justify enforcement action. However, if the accused motorist’s actions can be proven to be safe, reasonable and prudent for the prevailing conditions, the charge of speeding shall be dismissed by the court of jurisdiction.

Absolute Speed Limits:
Numerical speed limits, that if exceeded, shall be considered a speeding violation regardless of prevailing conditions. Any motorist proven to exceed an absolute speed limit shall be found guilty of a speeding violation.

85th Percentile Speed Limits:
Numerical speed limits based on a scientific survey of free flowing vehicle speeds. The speed at which 85 percent of the vehicles are traveling at, or below, is the 85th percentile speed.

Speed Zoning:
The process through which proper speed limits are determined and applied.

Speed Zoning Standards

Statutory speed limits shall be determined thorough valid traffic engineering speed surveys of a balanced sample for each road classification. The speed limits shall be set by administrative rule at the 85th percentile speed rounded to the next higher 5 mph increment. Statutory speed limits shall be prima facie speed limits.

The survey of free flowing speeds by road classification shall be carried out by the Department of Transportation (or equivalent agency). Statutory speed limits shall be adjusted to reflect the 85th percentile speed for each classification of roadway, rounded to the next higher 5 mph increment, no less than once every ten years.

Road classifications for which separate statutory speed limits are to be determined shall include:

  • Divided highway, uncontrolled access
  • Divided highway, limited access
  • Undivided rural highway, uncontrolled access
  • Urban arterial
  • Residential

Posted speed limits shall be based on 85th percentile speeds as determined by uniform traffic engineering surveys on the specific roadways to which they are applied.

Posted speed limits shall deviate from statutory speed limits only if the 85th percentile speed differs from the statutory limit.

Posted speed limits may be reduced five mph below the 85th percentile speed, as determined by a traffic engineering study, if accident rates for the subject roadway for the previous two years, exceed the state average accident rate by fifty percent, for roads of the same classification.

Posted speed limits shall be re-evaluated through a traffic engineering speed survey no less than once very ten years. The posted limit shall be adjusted to reflect the 85th percentile speed of the free-flowing traffic. The same process shall be employed anytime a street, road, or highway is substantially altered.

Speed Zoning Procedure

On a ten-year interval, minimum, each classification of roadway shall be surveyed to determine the 85th percentile speed representative of that classification of roadway. The survey shall be conducted during clear weather, on straight sections of dry roadway, absent construction, maintenance or visible enforcement activity.

Speed measurement shall be done in an unobtrusive, undetectable manner so as to obtain a sample of normal traffic speeds. If a daytime/nighttime speed limit differential is warranted, speed surveys should be conducted during both time periods.

If separate speed limits are believed warranted for different vehicle classifications, these vehicles should be the subject of a separate speed survey to determine their 85th percentile speed and subsequent speed limit.

Judicial Standards

The adjudication of speeding violations shall be based on the following standards as related to statutory and posted speed limits:

Vehicle operator may be charged with driving “too fast for conditions” or “reckless driving,” even if the numerical limit has not been exceeded.

The burden of proof is on the officer to document the conditions that required reduced speeds as well as the defendant’s failure to drive at speeds that reflected those conditions.

Exceeding prima facie speed limits is evidence of illegal speeding. Evidence entered on the defendant’s behalf that proves to the court that the defendant was not driving in an unsafe or irresponsible manner shall be considered a valid defense to justify dismissal of the speeding charge.

Any posted speed limit that is lower than the statutory speed limit for that classification of roadway shall not be enforceable unless it is supported by a valid traffic engineering study that documented a lower 85th percentile speed, or the need to reduce the posted speed limit by five mph.


  1. Cirillo, J.A., Interstate System Accident Research Study II Interim Report II. Public Roads, Vol 35, No 3, August 1969, pp. 71-75.
  2. Federal Highway Administration. Synthesis of Speed Zoning Practice. Report No. FHwA/RD-85/096. Washington, D.C. July 1985.
  3. Federal Highway Administration. Traffic Speed Trends. Washington, D.C. 1969-1975.
  4. Solomon, D., Accidents on Main Rural Highways Related to Speed, Driver, and Vehicle. Bureau of Public Roads (precursor to FHwA). July 1964.
  5. Federal Highway Administration. Effects of Raising and Lowering Speed Limits. Report No. FHwA/RD-92/084. McLean, VA. June 1996.
  6. Tignor, Samuel C. and Warren, Davey. Driver Speed Behavior on U.S. Streets and Highways. Institute of Transportation Engineers, 1990 Compendium of Technical Papers. Orlando, FL August, 1990.