Cal. Health & Safety Code § 39618

Refrigerated trailers shall be classified as mobile sources and shall be regulated by the state board on a statewide basis to prevent confusion concerning whether the trailers are stationary sources when not being driven and to prevent inconsistent regulation by districts of vehicles that are operated in more than one district. The state board shall develop regulations, on or before January 1, 2000, to achieve reductions in emissions attributable to the refrigerated trailers.