As someone who has lived beside too many busy roads, my instinct is to keep the cars quiet! I hate noisy cars. Still, we don't have to go to any extreme to be able to help blind people - a low hum would do the trick. In the long run, though, we shouldn't be 'driving' our cars anyway, they should drive themselves - the only people who will really WANT to drive when their onboard computer can do it for them, more safely, are exactly the sort of people who I hate driving by on the road (the SUV drivers who like to drive a few feet behind me at 70, and pass me just because I have more than 10 feet between me and the car ahead of me). When cars are driving themselves, blind people can just use small devices that connect to the 'road network' (used by cars to communicate to prevent accidents and determine ideal routes) and make a noise that alerts them of the presence of a car.