Germany has been using a Satellite based warning system (SatWaS) to alert citizens to any occurrence of natural or man-made disasters. The system sends signals to televisions and radios to warn the populace of the danger. Shortcomings are inherent to the system, not everyone may be listening to the radio or watching television. There is an option of installing a network of sirens, but that would be a very costly way to take. Researchers have now come up with a much simpler solution: use of car horns as warning systems.

An eCall emergency system will find its place in all cars manufactured 2010 onwards. Developed by researchers at the INT (Fraunhofer-Institut für Naturwissenschaftlich-Technische Trendanalysen), the system can be used to trigger horns of parked cars to warn of the trouble. The unit is capable of using GPS, so if required, the warnings can be localized to a certain area.

http://www.automotto.org/entry/germany-looking-at-car-horns-as-a-warning-for-disasters/