California Code, Public Resources Code - PRC § 25485

The department shall develop programs and undertake any necessary construction to establish, for the use of carpool vehicles carrying at least three persons, preferential lanes on major freeways in metropolitan areas where the total benefits to the carpool vehicles will bear a reasonable relationship to the total adverse effects on the remaining vehicles, as established on the basis of an engineering study. The department shall also permit such carpool vehicles to have access to preferential bus lanes established on major freeways, unless congestion seriously impeding the travel of buses will result or will present a serious traffic hazard.