There’s a message thread right now on the Model Railroader magazine forums with links to some You-Tube videos of a locomotive air horn mounted in a farmer’s field to scare away birds, and a person with horns mounted on the roof of his car. The message originator even says where you can buy them and what they cost.
http://cs.trains.com/mrr/f/88/t/209461.aspx
I’m not sure what the penalty would be for using them, but I have heard a semi driver here in Sierra Vista with them on his rig. You can hear them from a long way off.