The State covenants and agrees with the holders of any of the bonds issued by the Department under this chapter, for which there may be pledged the revenues of the Department or any part thereof, that so long as said bonds or obligations remain outstanding and unpaid and unless and until adequate provision is made by law for the protection of the persons advancing money upon such obligations, the State will not diminish or impair the power of the Department or of any successor thereof to operate or control any toll crossing constructed or maintained and operated by it, including the Delaware Memorial Bridge or to establish, levy and collect tolls, rates, rents or other charges in connection therewith.