California Code, Vehicle Code - VEH § 22358.8

(a) If the Department of Transportation or a local authority, after completing an engineering and traffic survey, finds that the speed limit is still more than is reasonable or safe, the Department of Transportation may or the local authority may, by ordinance, retain the currently adopted speed limit or restore the immediately prior adopted speed limit on a highway that is not a freeway if that speed limit was established with an engineering and traffic survey and if a registered engineer has evaluated the section of highway and determined that no additional general purpose lanes have been added to the roadway since completion of the traffic survey that established that speed limit.

(b) This section does not authorize a speed limit to be reduced by any more than five miles per hour from the currently adopted speed limit nor below the immediately prior speed limit.

(c) A peace officer shall issue only warning citations for violations of exceeding the speed limit by 10 miles per hour or less for the first 30 days that a lower speed limit is in effect as authorized by this section.