How Are Speed Limits Determined?

person driving

Everyone knows you are not supposed to go faster than the posted speed limit, but have you ever wondered how the speed limit is determined? Local speed limits are determined by municipalities using engineering and traffic surveys. Without a current and valid survey, an officer may be unable to prove a speed trap does not exist, and the officer's testimony may be considered incompetent (actual legal term).

California Vehicle Code section 627 defines an engineering and traffic survey as "a survey of highway and traffic conditions in accordance with methods determined by the Department of Transportation for use by state and local authorities." It also lists the factors that must be taken into consideration when determining a speed limit.

These factors include:

  • Prevailing speeds as determined by traffic engineering measurements
  • Accident records
  • Highway, traffic, and roadside conditions not readily apparent to the driver
  • Residential density
  • Pedestrian and bicyclist safety

By using the above legal reasoning, the speed limit should be set appropriately to facilitate the orderly movement of traffic so that it is reasonable and safe.

The Department of Transportation traffic manual states: "When a speed limit is to be posted, it shall be established at the nearest 5 mph increment of the 85th-percentile speed of free-flowing traffic." The 85th-percentile speed "statistically represents one standard deviation above the average speed and establishes the upper limit of what is considered reasonable and prudent." The speed limit may be reduced if "roadside development results in traffic conflicts and unusual conditions which are not readily apparent to drivers." The conditions and justification for the lower speed need to be documented in the engineering and traffic survey.

If a Court determines that the posted speed limit is not justified by the 85th-percentile speed, as stated in the engineering and traffic survey, the speed limit is not warranted. Also, if radar or lidar were used, the speeding charge must be dismissed. Understanding how speed limits are set is crucial to defending speeding tickets. We at RPM Law have handled thousands of speeding tickets and have experience in reading engineering and traffic surveys. We are here to help contact us now at RPM Law. We protect people and their driving records every day.