Section 6. Highway revenue non-diversion. (1) Revenue from gross vehicle weight fees and excise and license taxes (except general sales and use taxes) on gasoline, fuel, and other energy sources used to propel vehicles on public highways shall be used as authorized by the legislature, after deduction of statutory refunds and adjustments, solely for:
     (a) Payment of obligations incurred for construction, reconstruction, repair, operation, and maintenance of public highways, streets, roads, and bridges.
     (b) Payment of county, city, and town obligations on streets, roads, and bridges.
     (c) Enforcement of highway safety, driver education, tourist promotion, and administrative collection costs.
     (2) Such revenue may be appropriated for other purposes by a three-fifths vote of the members of each house of the legislature.