Idaho Const. art. VII, § 17

On and after July 1, 1941 the proceeds from the imposition of any tax on gasoline and like motor vehicle fuels sold or used to propel motor vehicles upon the highways of this state and from any tax or fee for the registration of motor vehicles, in excess of the necessary costs of collection and administration and any refund or credits authorized by law, shall be used exclusively for the construction, repair, maintenance and traffic supervision of the public highways of this state and the payment of the interest and principal of obligations incurred for said purposes; and no part of such revenues shall, by transfer of funds or otherwise, be diverted to any other purposes whatsoever.