A horn is a sound-making device that can be equipped with motor vehicles, buses, bicycles, trains, trams and other types of vehicles. The vehicle operator uses the horn to warn others of the vehicle’s approach or the presence or to call attention to some hazard.
Motor vehicles, ships, and trains are required by law in some countries to have horns. Like trams, trolley cars, and streetcars, bicycles are also legally required to have an audible warning device in many areas, but not universally, and not always a horn. There is Hooter/Ambulance/Traffic/police/Fire alarm.