Ala. Code § 23-6-10

For the purpose of providing funds to enable the corporation to pay the principal of, premium, if any, and interest on any bonds issued by it under the provisions of this chapter, and to accomplish the purposes and objects of its creation, there hereby are irrevocably pledged to such purpose and appropriated so much as may be necessary for such purpose of the following: The State Department of Transportation’s portion of the four cent per gallon excise tax levied on gasoline, motor fuel, and lubricating oil under the provisions of Title 40, Chapter 17, Article 6. All moneys hereby appropriated and pledged shall constitute a sinking fund for the purpose of paying the principal of, premium, if any, and the interest on bonds of the corporation.