For the formation, in such counties, of road districts for the care, maintenance, repair, inspection and supervision only of roads, highways and bridges; and for the formation, in such counties, of highway construction divisions for the construction only of roads, highways and bridges; for the inclusion in any such district or division, of the whole or any part of any incorporated city or town, upon ordinance passed by such incorporated city or town authorizing the same, and upon the assent to such inclusion by a majority of the qualified electors of such incorporated city or town, or portion thereof, proposed to be so included, at an election held for that purpose; for the organization, government, powers and jurisdiction of such districts and divisions, and for raising revenue therein, for such purposes, by taxation, upon the assent of a majority of the qualified electors of such districts or divisions, voting at an election to be held for that purpose; for the incurring of indebtedness therefor by such counties, districts or divisions for such purposes respectively, by the issuance and sale, by the counties, of bonds of such counties, districts or divisions, and the expenditure of the proceeds of the sale of such bonds, and for levying and collecting taxes against the property of the counties, districts or divisions, as the case may be, for the payment of the principal and interest of such indebtedness at maturity; provided, that any such indebtedness shall not be incurred without the assent of two thirds of the qualified electors of the county, district or division, as the case may be, voting at an election to be held for that purpose, nor unless before or at the time of incurring such indebtedness provision shall be made for the collection of an annual tax sufficient to pay the interest on such indebtedness as it falls due, and also for a sinking fund for the payment of the principal thereof on or before maturity, which shall not exceed forty years from the time of contracting the same, and the procedure for voting, issuing and selling such bonds shall, except in so far as the same shall be prescribed in such charters, conform to general laws for the authorizing and incurring by counties of bonded indebtedness, so far as applicable; provided, further, that provisions in such charters for the construction, care, maintenance, repair, inspection and supervision of roads, highways and bridges for which aid from the State is granted, shall be subject to such regulations and conditions as may be imposed by the Legislature.