Every motor vehicle when operated upon a highway shall be equipped with a horn in good working order capable of emitting sound audible under normal conditions from a distance of not less than two hundred feet. Except as otherwise provided in this section, it shall be unlawful for any vehicle to be equipped with or for any person to use upon a vehicle any siren, exhaust, compression, or spark plug whistle or for any person at any time to use a horn, otherwise than as a reasonable warning, or to make any unnecessary or unreasonably loud or harsh sound by means of a horn or other warning device. Every police and fire department and fire patrol vehicle and every ambulance used for emergency calls shall be equipped with a bell, siren, or exhaust whistle.