The NMA's Model Speed Zoning Law

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Statutory standards and requirements for establishing speed limits.


Speed limits should represent the maximum safe and reasonable speed on a highway during good traffic and roadway conditions capable of being traveled by the normally competent vehicle operator in a typical motor vehicle. 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% to 90% 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. (See the federal study which proves this.)

The speed zoning statute being proposed 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 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.

85th Percentile Speed Limits: Numerical speed limits based on a scientific survey of free flowing vehicle speeds. The speed at which 85% 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 speed surveys for each road classification. The speed limits shall be set by administrative rule at the 85th percentile speed rounded to the next highest 5 mph increment.

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

Posted speed limits shall be based on 85th percentile speeds as determined by uniform traffic engineering surveys. Posted speed limits shall deviate from statutory speed limits only if the observed and measured 85th percentile speed differs from the statutory speed limit for that classification of road.

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 administratively adjusted to reflect the 85th percentile speed for each classification of roadway, rounded to the next highest 5 mph increment, no less than once every ten years.

Speed Zoning Procedure

On a five year interval, minimum, each classification of roadway shall be surveyed, using scientific sampling procedures, to determine the 85th percentile speed representative of that classification of roadway. Each survey shall be conducted during clear weather, on straight sections of dry roadway, absent construction, maintenance or visible law enforcement activity.

Speed measurement should 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 believed warranted, speed surveys should be conducted during both time periods.

If separate speed limits are believed warranted for large trucks or other 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:

Taken from the NMA Homepage.

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