Cal. Const., art. XIX, § 2

Revenues from taxes imposed by the State on motor vehicle fuels for use in motor vehicles upon public streets and highways, over and above the costs of collection and any refunds authorized by law, shall be deposited into the Highway Users Tax Account (Section 2100 of the Streets and Highways Code) or its successor, which is hereby declared to be a trust fund, and shall be allocated monthly in accordance with Section 4, and shall be used solely for the following purposes:

(a) The research, planning, construction, improvement, maintenance, and operation of public streets and highways (and their related public facilities for nonmotorized traffic), including the mitigation of their environmental effects, the payment for property taken or damaged for such purposes, and the administrative costs necessarily incurred in the foregoing purposes.

(b) The research, planning, construction, and improvement of exclusive public mass transit guideways (and their related fixed facilities), including the mitigation of their environmental effects, the payment for property taken or damaged for such purposes, the administrative costs necessarily incurred in the foregoing purposes, and the maintenance of the structures and the immediate right-of-way for the public mass transit guideways, but excluding the maintenance and operating costs for mass transit power systems and mass transit passenger facilities, vehicles, equipment, and services.