It passed in 2010 - vehicles when travelling under 18 mph. US "Pedestrian Safety Act" (and you thought there was "gridlock") to be enacted in 5 years.
http://www.nhtsa.gov/staticfiles/rulemaking/pdf/Quiet_Vehicles_NPRM.pdf is the nhtsa's plan - detailing you had 60 days to comment. (the notice was posted in the basement, behind a door signed "Beware of leopard")
Wikipedia:
Japan issued guidelines for such warning devices in January 2010 and the U.S. approved legislation on December 2010.[1][2]
The European Parliament approved legislation that requires the mandatory use of "Acoustic Vehicle Alerting Systems" for all new electric and hybrid electric vehicles within 5 years.[3]
NHSTA STATEMENT ON THE MINIMUM SOUND REQUIRMENT:
U.S. Department of Transportation Proposes New Minimum Sound Requirements for Hybrid and Electric Vehicles Monday, January 7, 2013
Proposal Would Allow All Pedestrians to Detect Vehicles that Do Not Make Sound
WASHINGTON – As required by the bipartisan Pedestrian Safety Enhancement Act of 2010 (PSEA), the U.S. Department of Transportation’s National Highway Traffic Safety Administration (NHTSA) is proposing that hybrid and electric vehicles meet minimum sound standards in order to help make all pedestrians more aware of the approaching vehicles.
“Safety is our highest priority, and this proposal will help keep everyone using our nation’s streets and roadways safe, whether they are motorists, bicyclists or pedestrians, and especially the blind and visually impaired,†said U.S. Transportation Secretary Ray LaHood.
Electric and hybrid vehicles do not rely on traditional gas or diesel-powered engines at low speeds, making them much quieter and their approach difficult to detect. The proposed standard, Federal Motor Vehicle Safety Standard No. 141, would fulfill Congress’ mandate in the Pedestrian Safety Enhancement Act that hybrid and electric vehicles meet minimum sound requirements so that pedestrians are able to detect the presence, direction and location of these vehicles when they are operating at low speeds.
“Our proposal would allow manufacturers the flexibility to design different sounds for different makes and models while still providing an opportunity for pedestrians, bicyclists and the visually impaired to detect and recognize a vehicle and make a decision about whether it is safe to cross the street,†said NHTSA Administrator David Strickland.
The sounds would need to be detectable under a wide range of street noises and other ambient background sounds when the vehicle is traveling under 18 miles per hour. At 18 miles per hour and above, vehicles make sufficient noise to allow pedestrians and bicyclists to detect them without added sound. Each automaker would have a significant range of choices about the sounds it chooses for its vehicles, but the characteristics of those sounds would need to meet certain minimum requirements. In addition, each vehicle of the same make and model would need to emit the same sound or set of sounds.
NHTSA estimates that if this proposal were implemented there would be 2,800 fewer pedestrian and pedalcyclist injuries over the life of each model year of hybrid cars, trucks and vans and low speed vehicles, as compared to vehicles without sound.
NHTSA will send the proposal to the Federal Register today. Upon publication, the public will have 60 days to submit comments on this NHTSA action.