They're so quiet they're dangerous. That's one of the problems the automotive industry will have to overcome for its all-electric vehicles and hybrids that often run on electric power only.

The National Highway Traffic Safety Administration (NHTSA) said Monday that it is proposing minimum sound standards for hybrid and electric vehicles as a way to make pedestrians more aware of them as they approach.

Because those vehicles don't rely on traditional gas or diesel-powered engines at low speeds, they tend to be much quieter, making them hard to hear given the ambient street noise, says the Detroit Free Press. That's why The proposed standard -- mandated in the Pedestrian Safety Enhancement Act -- would require that the vehicles are detectable in a wide range of street noises whenever traveling under 18 miles per hour.

At 18 miles per hour and faster, the vehicles apparently make enough noise that pedestrians and bicyclists can hear them without added sound.

NHTSA said every automaker will have "a significant range of choices" about what sounds it picks for its vehicles, as long as it meets certain minimum requirements. The agency is sending its proposal to be listed in the Federal Register and the public will have 60 days to comment.

They even put out a series of sample sounds, which seem to sound a lot like other vehicles.

You can hear them at NHTSA-approved Sample Sounds.

The regulation requires that whatever sound is used, it increase in volume and pitch depending on the speed. And hybrid and electric vehicles will need some kind of sound when they are idling and in reverse as well as moving forward, NHTSA said.

If the proposed rule is finalized in a timely fashion, a 3-year phase-in could begin in September 2015.

"Safety is our highest priority, and this proposal will help keep everyone using our nation’s streets and roadways safe," said U.S. Transportation Secretary Ray LaHood.