23 C.F.R. § 650.113
(a) A proposed action which includes a significant encroachment shall not be approved unless the FHWA finds that the proposed significant encroachment is the only practicable alternative. This finding shall be included in the final environmental document (final environmental impact statement or finding of no significant impact) and shall be supported by the following information:
[44 FR 67580, Nov. 26, 1979, as amended at 48 FR 29274, June 24, 1983]