California Code, Health and Safety Code - HSC § 43806

On or before January 1, 1993, the state board shall adopt emission standards and procedures applicable to new engines used in publicly owned and privately owned public transit buses, and shall make the standards and procedures effective on or before January 1, 1996. The standards shall consider the engine and fuel as a system and shall reflect the use of the best emission control technologies expected to be available at the time the standards and procedures become effective. In adopting standards, the state board shall consider the projected costs and availability of cleaner burning alternative fuels and low-emission vehicles compared with other air pollution control measures.