Mobile county may become indebted and may issue bonds for the construction or improvement of concrete or better than concrete surfaced public roads, and concrete or better than concrete public bridges in said county, and for the construction of bridges and roadway necessary to provide a public road for vehicular travel between the highlands of Mobile and Baldwin counties, in an amount not to exceed six and one-half per centum of the assessed value of the property situated in Mobile county. To pay said indebtedness and interest thereon, Mobile county may levy and collect an annual tax on said property not to exceed one-half of one per centum of said value. The indebtedness, the bonds and the tax authorized hereby shall be in addition to those authorized by the Constitution of Alabama prior to the adoption of this amendment. But no such additional indebtedness shall be created, and no such additional bonds shall be issued, and no such additional tax shall be levied, until each improvement or construction proposed to be built thereby, its approximate location, estimated cost and time of completion, and the amount of the proposed increase, shall have been determined upon and made public by the board of revenue and road commissioners of Mobile county, and the proposed increase of indebtedness or issue of bonds or tax therefor shall have been first authorized by a majority vote by ballot of the qualified voters of Mobile county voting upon such proposition.