You also can get 12V "A-woo-gaa" horns, not so much for safety but more for fun.

A guy was working on a kit that had 2 car horns at different pitches in a special casing that mounted to the bike frame, so that you could charge and dismount easily making swapping it between bikes as easy as an airzound. I don't know how far the kickstarter got but you can check it out here:

http://www.kickstarter.com/projects/...s-for-cyclists