23 C.F.R. § 1215.3
As used in this part:
FHWA means the Federal Highway Administration.
Motor vehicle means any vehicle driven or drawn by mechanical power manufactured primarily for use on public highways, except any vehicle operated exclusively on a rail or rails.
NHTSA means the National Highway Traffic Safety Administration.
Passenger vehicle means a motor vehicle which is designed for transporting 10 individuals or less, including the driver, except that such term does not include a vehicle which is constructed on a truck chassis, a motorcycle, a trailer, or any motor vehicle which is not required on the date of the enactment of this section under a Federal motor vehicle safety standard to be equipped with a belt system.
Safety belt means, with respect to open-body passenger vehicles, including convertibles, an occupant restraint system consisting of a lap belt or a lap belt and a detachable shoulder belt; and with respect to other passenger vehicles, an occupant restraint system consisting of integrated lap shoulder belts.
Secretary means the Secretary of Transportation.
23 C.F.R. §1215.3