169.68 Horn, siren.
(a) Every motor vehicle when operated upon a highway must 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 200 feet. However, the horn or other warning device must not emit an unreasonably loud or harsh sound or a whistle. The driver of a motor vehicle shall, when reasonably necessary to insure safe operation, give audible warning with the horn, but shall not otherwise use the horn when upon a highway.
(b) A vehicle must not be equipped with, and a person shall not use upon a vehicle, any siren, whistle, or bell, except as otherwise permitted in this section.
(c) It is permissible, but not required, for any commercial vehicle to be equipped with a theft alarm signal device, so arranged that it cannot be used by the driver as an ordinary warning signal.
(d) All authorized emergency vehicles must be equipped with a siren capable of emitting sound audible under normal conditions from a distance of not less than 500 feet and of a type conforming to the federal certification standards for sirens, as determined by the General Services Administration. However, the siren must not be used except when the vehicle is operated in response to an emergency call or in the immediate pursuit of an actual or suspected violator of the law, in which latter events the driver of the vehicle shall sound the siren when necessary to warn pedestrians and other drivers of the vehicle's approach.