The government today made it official – those quiet electric and hybrid cars should sound more like – well – more like their conventional gas-powered cousins.

The problem is that at lower speeds the newer, environmentally-friendly vehicles just don’t make much of a sound. They’re so quiet that pedestrians may not even realize the cars are there. And that’s a safety problem.

A previous study found that hybrid electric vehicles are more than twice as likely to be involved in a pedestrian accident than conventional cars, when the vehicles are backing out, slowing or stopping, starting up in traffic, or going into or out of a parking space or driveway.

So Congress asked the Department of Transportation to do something about it. Today, the National Highway Traffic Safety Administration and the Volpe National Transportation Center, based in Massachusetts, issued their final report, detailing just what should replace those sounds of silence.

Researchers concluded the best solution was to ensure that electric and hybrid vehicles emit the same sound as vehicles with an internal combustion engine. They determined the sound should change as the car speeds up and slows down – to give pedestrians and other motorists even better audio clues about how the vehicle is moving. How to get that manufactured sound? Researchers suggested actual recordings of conventional cars, or a digitally reproduced alternative. As for how loud those added sounds should be? The suggested decibel level has yet to be determined.

NHTSA Administration David Strickland, said, “Today the agency turned a major corner in its efforts to make hybrid and electric vehicles more detectable to pedestrians. Our new research will help us strike the right balance between quiet roadways and pedestrian safety as we work to develop and propose a new safety standard.”

By law, the government must start its final rule-making by next summer, with a final rule in place by January 2014. Makers of electric and hybrid vehicles will have three years to phase in the noise requirements. By September of 2017, all new hybrid and electric cars will have to turn up the decibels.

So enjoy the quiet, while it lasts.