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 five hundred feet and of a type approved by the Department, but such siren shall not be used except when such 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 event the driver of such vehicle shall sound such siren when necessary to warn pedestrians and other drivers of the approach thereof.