Value of Speed Zoning

Although comparative “before-and-after” studies indicate that speed limit signs actually have very little influence on the driver’s choice of speed, speed zoning is necessary and does serve a worthwhile purpose. Realistic speed zoning will serve to protect the public and to regulate the unreasonable behavior of an individual. Having recognized that normally careful and competent actions of a reasonable person should be considered legal, the Texas Legislature has passed legislation concerning speed zoning in order to assure this protection. If a speed zone is determined by the actions of the majority of drivers on a highway, then it is hoped that speed zoning will facilitate the orderly movement of traffic by increasing driver awareness of a reasonable and prudent speed.
Properly applied speed zoning can:
  • help drivers adjust their speed to the conditions
  • make enforcement easier by furnishing police officers with a reasonable indication of what is excessive speed
  • result in more motorists driving within the same speed range at each of the locations along the highway
  • reduce the frequency and severity of crashes when accompanied by enforcement.
The Michigan Department of Transportation made a study entitled “Comparison of Speed Zoning Procedures and Their Effectiveness” dated September 1992. The following are some of the study’s conclusions:
  • Posting speed limits within 5 miles per hour of the 85th percentile speed has a beneficial effect, although small, on reducing total crashes but has a major beneficial effect on providing improved driver compliance. (See Chapter 3,
    Speed Zone Studies
    , for a discussion on the 85th percentile speed.)
  • Posting speed limits more than 5 miles per hour below the 85th percentile speed does not reduce crashes and has an adverse effect on driver compliance.
  • Speed zoning should not be used as the only corrective measure at high crash locations in lieu of other safety improvements.
  • The use of radar to collect speed data appears to underestimate the 85th percentile speed by approximately 3 miles per hour.