Very cool. Sad but here in Ottawa County MI we do not have many overpasses at all. Very Very few of them and two I can think of are over waterways.....now how do you get a sailboat to honk?
Most boats have horns for safety purposes.
You don’t necessarily need an Overpass; a sidewalk or roadside with traffic going past will do! Most of our efforts were directed at the traffic in front of us, not under us.
Give it a thought - 1 person with a sign and a flag can make a HUGH difference! ;)
Fog horns if they are properly equipped and obeying the law.