Personally, I'm annoyed by even the sound the tires make on the road.
The wind noise is bad enough, too. But I can't get rid of either of those, without enclosing the whole bike in a heavy, hot, sound-damping shell, or putting earplugs in, both of which would be stupid because they remove my ability to hear the environment around me, and be able to tell what's happening by sound.
I sure don't want any of the sounds that I can eliminate be required to be added back in!
I still think the answer is not to add noises, but to quiet everything else down, and teach people to not wear earbuds when on roadways, paths, etc., since whatever they're listening to on them seems to melt their brain and prevent them from noticing even the loudest horns, tires screeching, etc.
I have seen so many people come THAT close to being run over because they weren't paying ANY attention to what goes on around them, often but not always while wearing earbuds or headphones. Cars zooming along on the roadway, and joggers just suddenly turn and jog right off into traffic for no explicable reason, without looking AT ALL. Same thing for walkers and even cyclists, but especially the joggers with headphones/earbuds.
I think a part of the problem is that they are listening to their own stuff, and everything outside of that becomes background noise that they pay no attention to. Even emergency vehicles' sirens, and extremely loud truck horns do not get their attention, as I generally see zero reaction to such things.
Sometimes I don't see any reaction to even visual things like cars suddenly turning into a driveway right in front of them. I have seen someone walk right into a line of cars that were turning continuously into a parking lot, and bounce right off the bumper of the next car coming in, which couldn't possibly have stopped in time (because many drivers do not slow down enough, if at all, when approaching a turn-off or entering driveways, so they cannot react to whatever turns out to be present there, whcih they typically can't see until they reach the turn). Pretty obviously they weren't even looking *or* listening, as many of the cars on the road were honking at the ones turning and "slowing them down".
People that are paying attention don't seem to have any problem avoiding being run over, at least in cases where the vehicles aren't going so fast that you couldn't have seen them coming anyway, especially where they are going against a traffic control, etc. I watch people in parking lots quite a bit, when I'm out riding, at the store, or even at work when I'm cashiering since I'm right at the front windows of the store. Only the oblivious ones come close to getting (or actually do get) hit. All the others stop short of getting in the way, and though it might seem close it's not. It doesn't make any difference if the vehicles are silent (like a regular bicycle speeding along on the sidewalk in front of the store doors) or very loud (like a monster-thumping-stereo car or a roaring motorcycle suddenly zooming right thru the stop sign), people paying attention seem able to easily avoid the collisions, while people that are not paying attention for whatever reason are either nearly hit or actually do get hit, a fair percentage of the time.
So I really don't think noises from a car or a bike are going to help most of the cases where a collision could occur between anyone or anything. Properly paying attention to the world around you is the only good way to avoid collisions, as far as I can tell.
JK.
I suppose hub motors could work well.
