When each and all of the bonds, interest coupons, and obligations of every nature whatsoever, for the payment of which the revenues of any given project or projects have been pledged, in whole or in part, either originally or subsequently, either primarily or secondarily, directly or indirectly, or otherwise, have been paid in full, or a sufficient amount for the payment of all such bonds and other obligations and the interest thereon to the maturity thereof shall have been set aside in trust for the benefit of such bondholders or other obligees, such project or projects shall henceforth be maintained, free from any and all rental consideration, by the department, if part of the state highway system; and, if a county road or an urban road, then by the political subdivision which under paragraph (4) of Code Section 32-10-9 had the duty to maintain the project prior to the cessation of rentals, provided that, upon the cessation of rentals upon any given project, the authority may convey by deed all right, title, and interest in and to such project to the department as part of the state highway system; and, if not part of the state highway system, then to the political subdivision having maintenance responsibility for the project under this Code section; and provided, further, that the department shall maintain and keep in repair such free project or projects as are a part of the state highway system.