of any bridge upon and across any dam pursuant to this section shall not be commenced unless and until the State in which such bridge is to be located, or the appropriate subdivision of such State, shall enter into an agreement with such agency and with the Secretary to construct, or cause to be constructed, with or without the aid of Federal funds, the approach roads necessary to connect such bridge with existing public highways and to maintain, or cause to be maintained, such approach roads from and after their completion. Such agreement may also provide for the design and construction
of such bridge upon and across the dam by such agency of the United States and for reimbursing such agency the costs incurred by it in the design and construction
of the bridge as provided in subsection (d) of this section. Any such agency is hereby authorized to convey to the State, or to the appropriate subdivision thereof, without costs, such easements and rights-of-way in its custody or over lands of the United States in its custody and control as may be necessary, convenient, or proper for the location, construction
, and maintenance of the approach roads referred to in this section including such roadside parks or recreational areas of limited size as may be deemed necessary for the accommodation of the traveling public. Any bridge constructed pursuant to this section upon and across a dam in the custody and jurisdiction of any agency of the United States, including such portion thereof, if any, as may extend beyond the physical limits of the dam, shall constitute and remain a part of said dam and be maintained by the agency. Any such agency may enter into any such contracts and agreements with the State or its subdivisions respecting public use of any bridge so located and constructed as may be deemed appropriate, but no such bridge shall be closed to public use by the agency except in cases of emergency or when deemed necessary in the interest of national security.