The sounds of silence could soon be getting a microphone.

Hybrid and electric cars, typically far quieter than their more conventional counterparts, could soon be required to make more noise while they're traveling down the road.

A rule proposed Monday by the National Highway Traffic Safety Administration would require that automakers install devices on hybrids and electrics that make additional noise when the car is traveling at or below 18 miles per hour, when it's stationary or in reverse.

Hybrids and electrics have sometimes proven problematic for pedestrians and bicyclists. Without the familiar hum of a gas or diesel engine, there are fewer audible cues that alert others to the presence and direction of the cars.

Hybrid vehicles are 1.38 times more likely to be involved in collisions with pedestrians at low speeds and 1.33 times more likely to be involved in a crash with a cyclist, according to NHTSA data.

"We believe that this difference in accident rates is mostly attributable to the pedestrians' inability to detect these vehicles by hearing them during these maneuvers," NHTSA officials wrote in their notice of public rule-making.

NHTSA estimates the new rule would prevent 2,790 collisions, and save 35 lives per year. The public has 60 days to comment on the proposed rule before administrators begin crafting the wording of the final rule.

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Sound-making devices would cost auto manufacturers approximately $35 per vehicle to install and would give enough information for "pedestrians, bicyclists and the visually impaired to detect and recognize a vehicle," NHTSA administrator David Strickland said, "and make a decision about whether it is safe to cross the street."

NHTSA tested EVs and cars powered by internal combustion engines in a number of scenarios, and found that, on average, it took pedestrians longer to detect hybrids and electrics in "high ambient sound" scenarios, which mimicked the background noise pedestrians might hear, for example, when walking along a busy road.

Pedestrians will still want to keep a sharp eye on the road – a final rule isn't expected until January 2014, and will be phased in over a three-year period that may not begin until September 2015.

U.S. Secretary of Transportation Ray LaHood hailed the proposal as one that keeps all road users safe, but especially for blind people.

For others, it doesn't hurt, but it's "not a game changer," said Robin Stallings, the executive director of BikeTexas.org, a nonprofit organization that advocates for bike safety at local, state and federal levels. "For bicyclists and the speed that bicyclists go, even when we can hear them coming, it's often going to be too late."

He stressed the most significant step transportation officials can take to protect bicyclists is speed-specific lanes that separate traffic, such as the bike lanes that have become popular in New York and Chicago. "We believe that's going to be an even better solution than cyclists looking over their shoulders," Stallings said.

The proposed rule fulfills a 2010 mandate from Congress that hybrid and electric vehicles meet a minimum sound requirement so they can be detected. The proposed rule gives manufacturers flexibility in choosing the sound and setting the decibel level, but does require that it be consistent for each make and model. The sound should be recognized as vehicle, the report recommends.

Some automakers have already been researching the problem and possible solutions. Nissan conducted a survey of customers of its all-electric Leaf to gauge what sounds they found acceptable and annoying, and also interviewed blind pedestrians to ask which sounds are most useful.

Administrators arrived at the 18-mph point because at higher speeds, tire noise, wind resistance and "other factors" eliminated the need for additional sound devices.