Does Safety Dictate Making Quiet Cars Loud?

NHTSA proposes sound standards to make quiet cars safer – Welcome to the FastLane: The Official Blog of the U.S. Secretary of Transportation
I was initially moderately annoyed by this; we have a Prius, really *like* that it adds so much less noise to our environs, and are frustrated that chronic headphone users and texters seemed to be driving public policy (we live in a college town). Now that I’ve both read the actual rationale and heard some if the sounds, I’m coming around. I actually sorta dig the eerie Jetson’s-hover-car sound of sample three. I’d like my future to start sounding a little more futuristic.
One lingering concern: I regularly get my bike going 18 to 20 mph; why isn’t that a concern? My chain and tires are quieter than our Prius going 10mph, and I can’t stop my bike nearly as quickly (or, at least I don’t think I can. Guess I’ll go do some trials once it thaws.)

Because hybrid and electric vehicles operate so quietly, particularly at low speeds, they are more difficult for pedestrians and cyclists to detect when a vehicle is coming. This problem is even bigger for the visually impaired who rely on sounds for guidance.
The Pedestrian Safety Enhancement Act sought to fix that, and yesterday’s proposal is the result of NHTSA enthusiastically taking up the challenge. The proposal is now open to public comment for the next 60 days, so stay tuned for an update later this year.
Under the new standards, vehicle sounds would need to be detectable under a wide range of street noises and other background sounds when a vehicle is traveling slower than 18 miles per hour. At that speed and above, vehicles make sufficient noise to allow pedestrians and bicyclists to detect them without added sound.