A bell or 6v car horn is not going to cut it you need something more like this AMAZING-Train Horns On A Bicycle.flv - YouTube
For a loud as heck horn, try one of these:
Airzound Bike Horn -- Delta Cycle

Air Horns from a train! They would look nice mounted in my trailer. 
don't pay road taxes (d) can't defend yourself or avoid their terror tactics.
