Where I live we have trams. Big, heavy, hard to stop, track-bound items to generally keep away from Anyway, they have a quite distinctive bell/ringer that really cuts through traffic noise. Trams being what they are, people are pretty much conditioned to reacting to them - even if there's no tram or tramline right there. A friend of mine had one of those bells in his car - very effective... Worked even on pedestrians, who otherwise tend to totally ignore cars/car horns.

/Michael