@kim @meganL @ascentale @skyfaller @bikenite Sounds incredibly challenging. Besides the matter of using your hands while riding, sign languages incorporate use of the space in front of you (e.g. you reference a person by name and then assign them a position, and you can point to that position to say "she" and so on) and facial expressions (statement vs yes-no question vs W-question are all eyebrows, at least in ASL) as core grammatical components. My ASL was never good enough to let me have an informed opinion on whether reading signs from behind would be a lot harder or only a little, but I suspect a lot. (Clever use of bike mirrors could help?) There are ways to get your point across with one hand in many cases, but even fingerspelling takes both hands in the British sign language family (including Australian and New Zealand sign languages). You could certainly communicate to some extent but it wouldn't be nearly as smooth as a face-to-face conversation.

Could still be better than shouting, depending on the distance and conditions.

I also want to know how well Morse code with a compatible tone could work. A bell with decent resonance would be too slow, I think, if you want to enunciate (uh, the Morse code equivalent anyway) and it could be distracting and upsetting to others who have no idea why you're using your bell so aggressively. Finding something to make an appropriately clear but not alarming noise could be tricky, in addition to only being able to communicate within groups that all know Morse, of course. But CW radio with a handlebar mounted key could be very doable, and if everybody had receivers that sent the tones to something mounted in their helmets or open-ear headphones... (it's on my bucket list to design a handlebar mounted CW QRP setup).