Washington Revised Code Title 46. Motor Vehicles § 46.37.380. Horns, warning devices, and theft alarms

(1) Every motor vehicle when 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 200 feet, but no horn or other warning device may 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 his or her horn but shall not otherwise use such horn when upon a highway.

(2) No vehicle may be equipped with nor may any person use upon a vehicle any siren, whistle, or bell, except as otherwise permitted in this section.

(3) It is permissible for any vehicle to be equipped with a theft alarm signal device so long as it is so arranged that it cannot be used by the driver as an ordinary warning signal. Such a theft alarm signal device may use a whistle, bell, horn, or other audible signal but shall not use a siren.

(4) Any authorized emergency vehicle may be equipped with a siren, whistle, or bell capable of emitting sound audible under normal conditions from a distance of not less than 500 feet and of a type conforming to rules adopted by the state patrol, but the siren shall 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 reasonably necessary to warn pedestrians and other drivers of its approach.

(5) Any organ transport vehicle may be equipped with a siren, whistle, or bell capable of emitting sound audible under normal conditions from a distance of not less than 500 feet and of a type conforming to rules adopted by the state patrol, but the siren shall not be used except when the vehicle is transporting a time urgent organ as defined in RCW 68.64.010, in which case the driver of the vehicle shall sound the siren when reasonably necessary to warn pedestrians and other drivers of its approach.