Cal. Sts. & High. Code § 942

Such boards may enact and enforce ordinances and regulations for the construction, improvement or maintenance of county highways, and for the protection, supervision, management, control, or use of such highways. Any such board may by a four-fifths vote of all its members let county equipment used in the maintenance and construction of county roads, when such equipment is not in use upon the roads under the jurisdiction of the board, and may charge for the use thereof a rental, and arrange the basis of compensation, in keeping with the general conditions prevailing in the county in which the transaction is made.