Well the horn would be a useful item depending on what you wanted the outcome to be.
Case in point. We had a problem with deer on the road and our company trucks hitting them causing lots of damage to the trucks. Shop foreman says, "Hey, lets put those whistle things on the bumpers to warn the deer and they will stay our of our way". Well after we install the deer whistles, the deer strike rate almost doubles. Seems the deer who had been grazing on the side of the road were startled by the whistle and jumped up and ran across the road in front of the trucks.
I always think about that before I honk the horn, especially an air horn. Some moron gets startled, turns the wrong way or swerves and smack. But I almost always reward the guy who thinks he can just merge in front of me with his brakes on (causing me to rearrange the inside of the coach) with a honk of the horn. Makes me wish I had a train horn.......