California Code, Health and Safety Code - HSC § 41231

(a) After a public hearing, the Mojave Desert district may adopt regulations to require operators of public and commercial fleet vehicles, when adding vehicles to, or replacing vehicles in, an existing fleet or when purchasing vehicles to form a new fleet, to purchase low-emission motor vehicles, and to require, to the maximum extent feasible or appropriate, that those vehicles be operated on a cleaner burning alternative fuel.

(b) For purposes of this section, “motor vehicle fleet” means 10 or more vehicles under common ownership or operation.