As a lad, I used to deliver medicines for the local pharmacy on my bike. I had a proper air horn in my bottle holder because of the number of dozy drivers I encountered.
It must be remembered that I was delivering morphine, laxatives, muscle sprays and suchlike to frail retired persons on a whole bungalow estate of over 300 properties packed full of frail retired persons.
Thus, there was a larger than average proportion of cap-wearing, throttle-thrashing, clutch-slipping myopic pensioners whose idea of a cyclist was Miss Marple - and the Margaret Rutherford Miss Marple at that. You can imagine the hazards as the weepy-eyed old codgers tried to reverse out of their drives, or edge out of the side roads into my legs and my bike. At least they all had travelling blankets in the Austin Allegro to put over my lifeless form if one ever managed to take me out good & proper.
The air horn came in very handy once when I was stationary in the middle of the road, signalling and waiting to turn right.
A Mini was coming straight across out of the road I was wanting to turn into. The driver looked left & right but did not look directly ahead at all, and surged towards me. I gave a mighty blast of the air horn and the car performed an emergency stop right out in the carriageway. The button on the air horn actually jammed on and I needed to pull the horn off the gas canister to get it to stop.
It was also entertaining to use on close passes because it was so damn loud it would shake the car doors, and the driver would usually ease out to give more room.
Nowadays I just shout though. Normally rude words. And avoid bungalow developments like the plague..