California Code, Vehicle Code - VEH § 21351

Local authorities in their respective jurisdictions shall place and maintain or cause to be placed and maintained such traffic signs, signals and other traffic control devices upon streets and highways as required hereunder, and may place and maintain or cause to be placed and maintained, such appropriate signs, signals or other traffic control devices as may be authorized hereunder or as may be necessary properly to indicate and to carry out the provisions of this code or local traffic ordinances or to warn or guide traffic.