The Transportation section conducts Engineering and Traffic surveys to justify posted speed limits so police can use radar for speed enforcement purposes. Speed limits cannot be set at random or by popular opinion. California Vehicle Code (CVC) requires the posted speed limits to be consistent with the Engineering and Traffic survey results for Police to use radar for speed enforcement purposes.
The survey takes into account prevailing speeds, collision history, roadside conditions not readily apparent to the driver, residential density and pedestrian and bicycle safety.
Generally, speed limits are set at the 5-mile-per-hour increment below the 85 th percentile speed as determined by a radar survey. The 85 th percentile speed is the speed at or below which 85% of the drivers are traveling.". For example, if it is determined through a radar survey of the street that 85% of the vehicles are traveling at or below 37 miles per hour, the speed limit could be set at 35 miles per hour, and enforced by police through the use of radar. The law also permits lowering the speed limit indicated by 85 th percentile speed by 5 mph based on the factors mentioned above. Thus the 35 mph speed limit indicated by the 85 th percentile speed could be lowered to 30mph but not to 25 mph, if the survey also identifies a high accident rate, adverse roadside conditions, bike/pedestrian safety issues or residential density.