The state is authorized to appropriate funds, and to sell and issue interest-bearing state bonds, in addition to those heretofore authorized and sold, in an aggregate principal amount not exceeding $25,000,000 for the purpose of aiding in the acquisition, construction, and improvement of public roads, highways, and bridges in the state; provided, that the proceeds derived from the sale of the bonds issued under the provisions of this amendment may be used only for supplying the state's share of the cost of acquiring, constructing, and improving public roads, highways, and bridges in the state in conjunction with the United States and toward the cost of which funds have heretofore been or may hereafter be allocated to the state under the provisions of any law of the United States now in effect or hereafter enacted. Bonds sold and issued under the provisions of this amendment may be issued at such time or times and in such denominations, numbers, and series, and shall mature at such time or times, and shall have such terms and conditions, as may be provided by law. Said bonds shall bear interest at a rate or rates not greater than three per centum (3%) per annum, payable semiannually, and shall be sold at not less than the face value thereof. Said bonds when issued shall be direct general obligations of the state, and for the prompt and faithful payment of the principal thereof and interest thereon the full faith and credit of the state are hereby irrevocably pledged. In addition thereto, there is hereby irrevocably pledged for payment of the principal of and interest on said bonds, pro rata and without priority of one bond over another, so much as may be necessary for said purpose of the proceeds of the gasoline excise tax heretofore pledged for payment of the public road and bridge bonds of the state of Alabama provided for in the amendment to the Constitution of Alabama known as article XXA [amendment No. 21] subject, however, to the prior pledges of said tax for payment of any bonds heretofore issued pursuant to law for which the said tax has heretofore been pledged.