A
vehicle horn is a sound-making device in an
automobile,
truck, or other vehicle. The vehicle operator uses the horn to warn others of the vehicle's approach or presence, or to call attention to some hazard. Automobiles, trucks, ships, and trains are required by law in some countries to have horns. Bicycles in many areas are also legally required to have an audible warning device, but not universally, and not always a horn.