Speed Limit Establishment

Setting Speed Limits

In practice, speed limits are normally set at the first five-mile per hour increment below the 85%ile speed, defined as that speed at or below which 85% of the traffic is moving. Any further reduction must be documented by and clearly justified by an Engineering and Traffic Survey.

Conditions for Signage

In residential neighborhoods, 25 mile per hour speed limit signs may be installed without an engineering and traffic survey so long as these conditions are met:

  • The street must be considered local on the Federal Highway Administration (FHWA) map.
  • There can only be one travel lane in each direction.
  • Uninterrupted length cannot be more than 1/2 mile; interruptions include official traffic control devices such as stop signs and traffic signals.
  • The width does not exceed 40 feet.
In all other areas the speed limit is set by an engineering and traffic survey which is done once every seven years for each segment of roadway.

Traffic Problems

When traffic problems occur, concerned citizens frequently ask why we don't lower the speed limit. There are widely held misconceptions that speed limit signs will slow the speed of traffic, reduce accidents, and increase safety. Most drivers drive at a speed that they consider to be comfortable, regardless of the posted speed limit. Before and after studies have shown that there are no significant changes in average vehicle speeds following the posting of new or revised speed limits. Furthermore, research has found no direct relationship between posted speed limits and accident frequency.

Realistic speed limits help preserve uniformity of speed. Uniformity of speed is valuable for a number of reasons:

  • It enables pedestrians to more accurately judge the speed of traffic.
  • It maintains consistency in traffic gaps for crossing traffic.
  • It makes unreasonable violators more obvious to enforcement personnel.
  • It reduces the possibility of conflict between faster and slower drivers.

Additional Information

Additional Information If you have further questions regarding the establishment of speed limits or a missing or needed speed limit sign, please call Traffic Engineering at 707-543-3814.

If you have concerns regarding the enforcement of speed limits please contact the Traffic Division of the Santa Rosa Police Department at 707-543-3636.