I have one and it works wonders . People running my stop arm are suddenly aware of what they did. Some have no clue. In country the student can hear it. Plus other benefits. What is your view.
Air horns can be fun. I don't buy them simply because around here we have a large population of retirees that don't want to be disturbed, so we only use horns for emergencies.
Seems like a spec for rural operations; like fog flashers, these wouldn't go over well in town. I'm sure my drivers would overuse them. When you have a hammer in your hand, everything looks like a nail...