D.C. Mun. Regs. tit. 18, r. 18-730
Each motor vehicle operated upon a highway shall be equipped with a horn in good working order and capable of emitting sound audible under normal conditions from a distance of not less than two hundred feet (200 ft.).
No horn or other warning device shall emit an unreasonably loud or harsh sound or a whistle.
The driver of a motor vehicle shall, when reasonably necessary to ensure safe operation, give audible warning with his or her horn, but shall not otherwise use the horn when upon a highway.
No vehicle, other than an authorized emergency vehicle as set forth in § 712, shall be equipped with nor shall any person use upon a vehicle any siren, whistle, or bell, except as otherwise permitted in this chapter.
The prohibition in § 730.4 relating to the use of a bell shall not be applicable to any street car, bicycle, or motor vehicle used by a person licensed under the License Act of July 1, 1902 (D.C. Code, Title 47, Chapter 23), as amended, who customarily uses a bell or bells in connection with the sale of services or wares.
Upon approval by the Director, any vehicle may be equipped with a sound device designed to be used solely as a theft alarm system which shall be so arranged that it cannot be used or controlled by the driver or other person for any purpose other than as an alarm system.
D.C. Mun. Regs. tit. 18, r. 18-730