The state is authorized to engage in the construction, improvement, repair and maintenance of public roads, highways and bridges in the state of Alabama. To this end and for this purpose the state is authorized to appropriate funds, and also to issue and sell interest-bearing negotiable state bonds in addition to those already authorized and sold under article XX, as an amendment to the Constitution of 1901, in an amount not to exceed the sum of twenty-five million dollars ($25,000,000.00); to be issued in such denominations, numbers and series, and maturing at such times as may be provided by law; all such bonds shall bear a rate of interest not greater than six per cent per annum, payable semi-annually, and shall be sold at a price not less than the par value thereof. The state highway commission or highway department shall locate, construct and maintain highways and state trunk roads so as to connect each county seat with the county seat of the adjoining county by the most direct or feasible route or by a permanent road, having due regard to the public welfare; and to connect the county seats of the several border counties at or near the state line with a public road in the border state. Provided that in counties which are divided into two or more judicial divisions in each of which regular terms of the circuit court are held, the places where said terms of court are held, shall likewise be connected with each other. It shall be the duty of the highway commission or highway department to equitably apportion among the several counties of the state the expenditure of both money and labor and the time or times of making such investment. Not less than one-quarter of a million dollars of the proceeds of these bonds shall be set aside and expended by the state highway commission in each county in the state. To create a sinking fund for the prompt and faithful payment of the principal and interest on these bonds and for the construction, maintenance and improvement of such public highways, roads and bridges, the legislature shall levy an excise tax, in addition to the levy made February 10, 1923, of two cents per gallon upon gasoline or any substitute therefor, or an adequate license or excise tax on any other motive power used to propel auto vehicles. Such bonds when issued shall be a direct obligation of the state and for the prompt and faithful payment of the principal and the interest thereon the full faith and credit of the state is hereby irrevocably pledged and such bonds shall be exempt forever from taxes of every kind.