Cal. Veh. Code § 21358

Where one or more business and residence districts are contiguous, or where, as authorized by this code, speed is to be restricted between two districts, either business or residence, or at the end of either thereof, speed restriction signs affecting traffic on other than state highways as required by Sections 21357 and 21359 need be erected and maintained only at the boundaries of the outside limits of the area in which speed is to be restricted.