Cal. Veh. Code § 8050
The Legislature declares that in enacting this article, it adheres to the principle that each state should have the freedom to develop the kind of highway user tax structure that it determines to be most appropriate to itself, that the method of taxation of interstate vehicles should not be a determining factor in developing its user tax structure and that annual taxes or other taxes of the fixed fee type which are not imposed on a basis that reflects the amount of highway use should be apportioned among the states on the basis of vehicle miles traveled within each of the states. If the department determines that apportionment of the taxes on the basis of vehicle miles for a particular fleet of vehicles is impractical, the department may require the taxes on the fleet to be apportioned on an equivalent basis other than miles, as determined by the department.
Ca. Veh. Code § 8050