23 U.S.C. § 315

Except as provided in sections 202(a)(5), 203(a)(3), and 205(a) of this title, the Secretary is authorized to prescribe and promulgate all needful rules and regulations for the carrying out of the provisions of this title. The Secretary may make such recommendations to the Congress and State transportation departments as he deems necessary for preserving and protecting the highways and insuring the safety of traffic thereon.