Excellent. I've thought of that one myself. You'll see I suggested a "courtesy horn" for that very reason.

Could we also have a sign that says something like "You drive like a blind-folded drunken monkey with one arm". Perhaps it could just be an LED board and we could type in a variety of phrases for quick and handy access.

"I am very impressed with the loudness of your speakers, why they even rattle my windows, how wonderful!"

"My, you sure showed me by passing me at that great rate of speed! You got to the red light several seconds prior to my arrival, you are quite the superior human!"

"Passing me on the right side, how creative! And if you kill someone, oh well..."

You might simply adopt the practice to manually turn on your emergency flashers for a couple of flashes. Truck drivers do something similar. When a semi passes another vehicle, it is customary to flash headlight (on & off, or high-low beams) to let the lead truck know their trailer is clear of your vehicle and can pull over into the righthand lane. The customary response is a flashing of the trailer running lights to thank the driver of the passed vehicle for helping them. Each trucker has their own unique flashing sequence.