Air horn?
I have an air horn, but I am a little ambivalent about it. It is so loud that I am afraid to use it in traffic because I think it will startle people and perhaps cause them to do something dangerously erratic. It definitely makes people jump and is really quite annoyingly loud. I think if I did it again I might consider one of the louder electric options that could be used with the existing mounts and wiring. I wish I could select which horn to use (I left the factory horn in the circuit.)
I was hoping that the loud horn would deter animals (deer, cattle, etc) that I might see on the road far in front of me, but though it gets their attention, they just stare at me, then only scatter as I get very close.