Setting Speed Limits

Example of speed limit signs on the roadway

Speed limits establish a reasonable and safe operating speed for a specific section of roadway.

Per California Vehicle code (CVC) Section 22349, the maximum speed limit on a multilane highway and two lane undivided roadway is 65 mph and 55 mph respectively.  Agencies conduct Engineering and Traffic surveys (E&TS) to post speed limits that are lower than these speed limits. Engineering and traffic surveys are also referred to as speed zones.

Speed limits are established by an E&TS, by considering the following:

  1. Prevailing speeds (or 85th percentile speeds).
  2. Collision History
  3. Highway, traffic, and roadside conditions not readily apparent to the driver.

Other factors that may be considered while developing E&TS are, business or residential density, pedestrian and bicyclist safety.

The most widely accepted method of determining the posted speed limit is to set the speed limit at what is called the “85th percentile speed”, which is the speed at or below which 85 percent of the traffic is moving.

The 85th percentile speed is the single most influential indicator of what is safe and reasonable, and it is used to determine the speed limit by rounding to the nearest 5 mph increment, and posting the speed limit at that speed.  If there are sufficient conditions not readily apparent to the driver, as determined by a registered engineer, the posted speed limit can be lowered by 5 mph, and such a decision must be clearly documented in the E&TS.

More information on speed limits can be found below: