I don't think a horn is useful on the bike, honestly. You will rarely need a horn on the bike, and in situations where you do need to get someone's attention in a car, they will be close enough to you to shout out at them pretty easily.johnny847 wrote:Lots of good discussion here.
I'm a bike commuter, and I'd like to chime in with this suggestion: Get a bike horn. Like this one: http://www.amazon.com/Delta-Airzound-Bi ... 000ACAMJC/. It's a air canister connected to a horn. The canister can be pumped with a normal bike pump (Schrader valve). It's rated for about 30 half second uses on a full canister.
It's as loud as a car horn.
Visibility is the key to coexisting with cars on the road. If you're riding in dusk/dark, get the brightest rear light you can afford, and make sure those batteries are topped off. Hi-vis clothing or preferably riding in groups during daytime to add visibity.