I always thought that a miniature recorder/speaker that you attach to the handlebars would be ideal. Something the size of a bell or speedo with enough volume to be head from a distance. Record your message or perhaps 2 or 3 messages and when needed just press the appropriate bottom.
I can think of a 1000 appropriate (Or inappropriate) messages you could use. Record something from a movie or tv show, an animal sound, screeching car brakes, etc., or just make up your own message using only your voice.
For instance, a sequential message might start with, "Bicycle behind you passing left." If no reaction, "Watch out for the bike behind you!" at slightly higher volume. If still to reaction, " Get the (
insert your choice of word or words here) out of the way!" at full volume.
We have cell phones with more computing power than NASA had available for the lunar landings, surely someone like chinamart could produce such a miniature digital device that could be sold for a few bucks.
You could even add a forward projecting blinking light beam that would provide a visual clue for hearing impaired.
If you've never seen the movie Google Porky's Pig Horn sound
