Air horns are great to get attention heading into an intersection, round about or towards traffic lights.
these days with airconditioned cars fitted with nice radios, cd players, mp3 players, I think better lights would be better - i.e. head light flashers on high beam, more forward facing lights at rear vision mirror level!!!!!!!!!!
Also better at night, theres nothing worse than cruising through suburbia at night with the sirens going waking every man and their dog - not only is it annoying for the crews but does not serve to do mush other than wake the entire neighborhood up and them a little annoyed!