14 C.F.R. § 151.89 – Roads.

(a) Federal-aid Airport Program funds may not be used to resolve highway problems. Only those airport entrance roads that are definitely needed and are intended only as a way in and out of the airport are eligible.
(b) The construction, alteration, and repair of airport roads and streets that are entirely within the airport boundaries are eligible under the program, if needed for operating and maintaining the airport. In the case of an entrance road, a strip right-of-way joining the main body of the airport to the nearest public road may be considered a part of the normal boundary of the airport if—
1. (1) Adequate title is obtained;
2. (2) It was acquired to provide an airport entrance road and was not, before the existence of the airport, a public thoroughfare;
3. (3) The entrance road is intended only as a way in and out of the airport; and
4. (4) The entrance road extends only to the nearest public highway, road, or street.
(c) An entrance road may be joined to an existing highway or street with a normal fillet connection. However, acceleration-deceleration strips or grade separations are not eligible.
(d) Offsite road or street relocation needed to allow airport development or to remove an obstruction, and is not for entrance road purposes, is eligible.
(e) Appendix G sets forth typical eligible and ineligible items of road construction covered by this section.