A common belief is that posting a speed limit will influence drivers to drive at that speed. The facts indicate otherwise. Research conducted in many parts of this country over a span of several decades has shown that drivers are influenced more by the appearance of the highway itself and the prevailing traffic conditions than by the posted speed limit. California's Basic Speed Law requires that:
"No person shall drive a vehicle upon a highway at a speed greater than is responsible or prudent having due regard for weather, visibility, the traffic on, and the surface and width of the highway, and in no event at a speed which endangers the safety of persons or property."
Under California law, the maximum speed limit for any passenger vehicle is now 65 miles per hour (MPH). All other speed limits are called prima facie limits, which "on the face of it" are safe and prudent under normal conditions. Certain prima facie limits are established by law and include the 25 MPH limit in business and residential districts, the 15 MPH limit in alleys, at blind intersections and blind railroad crossings and a part-time 25 MPH limit in school zones when children are going to and from school. These speeds are not always posted but all California motorists are required to know these 10, 20, and 30 mile per hour speed laws.
Intermediate speed limits between 25 and 55 miles per hour may be established by local authorities on the basis of traffic engineering surveys. These surveys include an analysis of roadway conditions, accident records, and the prevailing speed, or 85th percentile, of prudent drivers. If speed limit signs are posted for a lower limit than is considered reasonable, many drivers will simply ignore the signs. At the same time, other drivers will stay within the posted limits. This generally increases the conflicts between faster and slower drivers, reduces the gaps in traffic through which crossings could be made safely and increases the difficulty for pedestrians to judge the speed, of approaching vehicles. Studies have shown that where uniformity of speed is not maintained, accidents generally increase.