§ 393.81 Horn.

PART 393—PARTS AND ACCESSORIES NECESSARY FOR SAFE OPERATION, last amended 2016-12-12

Every bus, truck, truck-tractor, and every driven motor vehicle in driveaway-towaway operations shall be equipped with a horn and actuating elements which shall be in such condition as to give an adequate and reliable warning signal.

Official text from the eCFR. The eCFR is an authoritative but unofficial editorial compilation; the Federal Register is the legal source.