49 U.S.C. § 31504
Identification of motor vehicles
(a) General Authority.—
The Secretary of Transportation may—
(1)
issue and require the display of an identification plate on a motor vehicle used in transportation provided by a motor private carrier and a motor carrier of migrant workers subject to section 31502(c) of this title , except a motor contract carrier; and
(2)
require each of those motor private carriers and motor carriers of migrant workers to pay the reasonable cost of the plate.
(b) Limitation.—
A motor private carrier or a motor carrier of migrant workers may use an identification plate only as authorized by the Secretary.
( Pub. L. 97–449 , Jan. 12, 1983 , 96 Stat. 2439 , § 3104; renumbered § 31504 and amended Pub. L. 103–272, § 1(c) , (e), July 5, 1994 , 108 Stat. 745 , 1030.)