49 U.S.C. § 32511

1. (a)Except as provided in this section, a State or a political subdivision of a State may prescribe or enforce a bumper standard for a passenger motor vehicle or passenger motor vehicle equipment only if the standard is identical to a standard prescribed under section 32502 of this title.
2. (b)This chapter and chapter 301 of this title do not affect the authority of a State to enforce a bumper standard about an aspect of performance of a passenger motor vehicle or passenger motor vehicle equipment not covered by a standard prescribed under section 32502 of this title if the State bumper standard—
   1. (1)does not conflict with a standard prescribed under chapter 301 of this title; and
   2. (2)was in effect or prescribed by the State on October 20, 1972.
3. (c)The United States Government, a State, or a political subdivision of a State may prescribe a bumper standard for a passenger motor vehicle or passenger motor vehicle equipment obtained for its own use that imposes additional or higher standards of performance than a standard prescribed under section 32502 of this title.