The Toyota Prius and other hybrid cars have become quite popular in Berkeley, replacing the Volvo and VW van as the car of choice. I think this is great, more power to them (har har).
The problem is, I walk and ride my bike around Berkeley a lot, and I have two little kids, and these things are a menace. When they're driving slowly, say through parking lots, or coming up to a stop light, they're almost completely silent, only their tires making any sound. As a pedestrian I constantly find myself jumping to discover one of these things coming out of nowhere.
It seems to me they should require some kind of signature sound to let those of us using people-power to detect when they're nearby.