I've had like 4 cyclists ride into my Prius. It's like a magnet for the unaware.
Better approach: Install a small wireless sender in the car that broadcasts at a given frequency.
Then allow people who want a minimum noise level to buy a small receiver that broadcasts those signals (or another type of indication signal) to a headset.
This allows you to keep the noise level down and at the same time provides "notifications" to the visually impaired.
1. Blind folks generally rely on directional sound. Knowing a car's nearby is better than nothing, but you can't tell how fast it's approaching you, from what direction.
2. In a city, it'd never stop beeping at you.
2. Beeping was just an example. Just "encode" the information as motor noise and it will sound more "natural".
Dumb idea.