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Speed Limits

Have you ever wondered how speed limits are set or why some of them are not reduced "to slow traffic down"? Well, the answers are based partly in State law and partly in human nature. In addition to the basic speed laws described elsewhere, the following addresses how speed limits are established and what role the State plays in this process.

The Basics

For speed regulations to be reasonable and effective, they must be based on the following considerations:

  • Longstanding experience indicates that the great majority of drivers normally travel at a speed that is reasonable and prudent, continually assessing and compensating for physical roadway features and variable traffic conditions.
  • The normally careful and competent actions of reasonable drivers should be considered legal.
  • Speed limits can not be successfully enforced without voluntary compliance by a majority of drivers.

The Procedures

The State Legislature has recognized these principles and passed laws (contained in the Vehicle Code) mandating that cities follow certain procedures to establish local speed limits. These procedures require the City to conduct a survey of street and traffic conditions, considering collision history, conditions such as sight distance or roadway vertical alignment that may not be readily apparent to motorists, and prevailing speeds. These speed surveys are conducted by trained engineering staff. Accident records are reviewed, accident rates are calculated and compared with Countywide averages, a complete field review is performed and, for each street segment, a statistical sample of vehicle speeds is collected with a radar gun from an unmarked car. The speed data is analyzed to determine variables such as average speed and prevailing speed. The prevailing speed is also known as the 85th percentile speed, because it is the speed that is exceeded by only 15% of the traffic.

Why Not Set a Lower Limit? State law requires the speed limit to be set at the 5 mph increment nearest the prevailing speed, unless there are particularly extenuating circumstances relating to pedestrians, bicycles, or a pattern of speed-related collisions. Although residents occasionally request that speed limits be reduced, a further 5 mph speed limit reduction would frequently result in the following:

  • The reduced speed limit would make the majority of drivers illegal.
  • The reduced speed limit could be effectively unenforceable, because the use of radar could constitute a speed trap (which is forbidden by State law).
  • The reduced speed limit would not significantly affect speeds or accident rates, as consistently demonstrated by before-and-after studies.

For these reasons, the City establishes realistic speed limits that are based on prevailing speeds, are consistent with State law, and are enforceable. The City Traffic Engineer submits the speed study to the City Council for review and approval. The City Council then adopts a Resolution formally establishing the speed limits, and the speed limits become enforceable following posting of appropriate signs. If you would like more information about speed limits, feel free to contact the City Traffic Engineer.