Veh. Code, § 22365

Notwithstanding any other provision of law, any county or city, which is contained, in whole or in part, within the South Coast Air Quality Management District, may, if the county or city determines that it is necessary to achieve or maintain state or federal ambient air quality standards for particulate matter, determine and declare by ordinance a prima facie speed limit that is lower than that which the county or city is otherwise permitted by this code to establish, for any unpaved road under the jurisdiction of the county or city and within the district. That declared prima facie speed limit shall be effective when appropriate signs giving notice thereof are erected along the road.