49 C.F.R. § 393.81 Horn.
Title 49 - Transportation
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.
Title 49: Transportation
PART 393—PARTS AND ACCESSORIES NECESSARY FOR SAFE OPERATION
Subpart G—Miscellaneous Parts and Accessories
§ 393.81 Horn.