Local jurisdictions have authority to establish reduced speed limits on the basis of engineering and traffic surveys (CVC 22358). Such surveys must include an analysis of roadway conditions, accident records and a sampling of the prevailing speed of traffic, (CVC 627). Other factors may be considered, but an unreasonable speed limit, which is called a speed trap, may not be established, (CVC 40802).
The Legislature has declared a strong public policy against the use of speed traps, to the extent that citations issued where a speed trap is found to exist are likely to be dismissed, particularly if radar enforcement methods are used (CVC 40803 - 40805).
A provision of the vehicle code that California Courts have generally considered very important reads, "It is the intent of the Legislature that physical conditions such as width, curvature, grade and surface conditions, or any other condition not readily apparent to a driver, in the absence of other factors, would not require special downward speed zoning, as the basic rule of section 22350 is sufficient regulation as to such conditions" (CVC 22358.5).
The words, It is the intent of the Legislature, are intended to get the attention of traffic engineers and local jurisdictions in setting and maintaining local speed limits. Such speed limits must be set carefully, as justified by appropriate factors, to avoid making such limits unenforceable.