Streets and Highways Code § 91.5

(a) The department may enter into an agreement to accept funds, materials, equipment, or services from any person for maintenance or roadside enhancement, including the cleanup and abatement of litter, of a section of a state highway. The department and the sponsoring person may specify in the agreement the level of maintenance that will be performed.

(b) The director may authorize a courtesy sign. These courtesy signs shall be consistent with existing code provisions and department rules and regulations concerning signs.