California Code, Vehicle Code - VEH § 22360

(a) Whenever a local authority determines upon the basis of an engineering and traffic survey that the limit of 65 miles per hour is more than is reasonable or safe upon any portion of a highway other than a state highway for a distance of not exceeding 2,000 feet in length between districts, either business or residence, the local authority may determine and declare a reasonable and safe prima facie limit thereon lower than 65 miles per hour, but not less than 25 miles per hour, which declared prima facie speed limit shall be effective when appropriate signs giving notice thereof are erected upon the street or highway.

(b) This section shall become operative on the date specified in subdivision (c) of Section 22366.