49 U.S.C. § 31147 — Limitations on authority

(a) Traffic Regulations — This subchapter does not give the Secretary of Transportation power to make general traffic safety rules or to override state traffic laws. The one exception: the Secretary may make traffic rules on subjects that were already covered by parts 390-399 of title 49 of the Code of Federal Regulations, as those parts stood on October 30, 1984. (b) Regulating the Manufacturing of Vehicles — This subchapter also does not let the Secretary regulate how commercial motor vehicles are built, for any reason — including fuel economy, safety, or emissions.