Well, we have a situation now where all cars make noise when they're active. That's a side effect, but it benefits blind people greatly.
Back in the 1890's, when most cars were electric and most other vehicles were also silent (horses!) blind people were out of luck. They were shut-ins and had no rights and no choices. Now, we want the disabled to be full members of society. That's a good thing.
Having cars make noise may seem silly, but it really helps the blind. It doesn't have to be loud or obnoxious noise, notice that many existing cars are not that loud. It's easy enough to do... why not do it?
Besides, you'll never live to see quiet streets. There will be fuel-engine vehicles for a long, long time to come, and then there will always be teenage boys who need to make a statement with their vehicle.
John W - downloadable enginetones... rock on. I'm going for Alco 539 myself. http://www.soundtraxx.com/dsd/tsunami/index.php?p=tsunamisound.php
And mrvadebo... there's your cost. $65 in low quantity.