Piggybacking on "lvrr325's" good answer regarding air horns,
(Courtesy of Yahoo Answers)
"First off there is a lot more than just a horn for the big truck horns, their ran from an air compressor or off a reserve air tank and cost a lot to get the true big truck air horn sound. Second, a loud horn is not necessary for a small car, the concept behind the air horn is it takes a fully loaded big truck three times the distance to stop, so they are required three times the audible warning capability in case of of brake failure or stupid drivers. that is why trains have such a mega horn system, it takes them over a mile to come to a stop...a louder horn is cool but is also annoying, and because your horn is louder, doesnt make your point more valid. New Yorkers seem to be the only people who use the horns because they think people really care what they think, but we dont, horn honkers really blow...just learn to drive for other people and keep your cool, people make mistakes, and a horns dont correct it...if they are close enough to hurt you, they are close enough to hear you."