When the Secretary determines it to be in the public interest, the Secretary may designate, as System unit approach roads, roads whose primary value is to carry System unit travel and that lead across land at least 90 percent owned by the Federal Government and that will connect the highways within a System unit with a convenient point on or leading to the National Highway System.

An approach road designated for a System unit shall be treated as a supplementary part of the highway system of the System unit.

The Secretary may construct, reconstruct, and improve approach roads designated under paragraph (1) (including bridges) and enter into agreements for the maintenance of the approach roads by State or county authorities or to maintain the approach roads when otherwise necessary.

Not more than $1,500,000 shall be allocated annually for the construction, reconstruction, and improvement of System unit approach roads.