The provision of this title shall apply to all unappropriated authorizations contained in prior Acts, and also to all unexpended appropriations heretofore made, providing for the expenditure of Federal funds on the following classes of highways: Forest highways, forest development roads and trails, park road, parkways, Indian reservation roads, refuge roads, public lands highways, and defense access roads. All such authorizations and appropriations shall continue in full force and effect, but hereafter obligations entered into and expenditures made pursuant thereto shall be subject to the provisions of this title.