Washington ? The age of the silent hybrid may be coming to an end.

Gas-electric hybrids, propelled by electric motors at low speeds, are well-known for their quiet ride and great mileage. But their silence isn’t always golden.

Some researchers and safety groups say that quiet operation — “hybrid creep” — can pose risks for unsuspecting pedestrians and the blind, who use sound cues.

Advocates for the blind have sought the addition of artificial noises in hybrids for several years, concerned that the expected sales growth of hybrids could lead to more pedestrian fatalities and injuries. Hybrids account for about 2 percent of new car sales each year but auto companies are expected to boost production in advance of tougher fuel efficiency standards this decade.

“This is an example of too much of a good thing,” said John Pare, executive director for strategic initiatives with the National Federation of the Blind. “Cars got quieter, that was good. Suddenly they got to be so quiet that it added an element of danger.”

The government’s auto safety agency said in a research report last year that hybrid vehicles are twice as likely to be involved in pedestrian crashes at low speeds compared with cars with conventional engines. The study by the National Highway Traffic Safety Administration examined circumstances in which the vehicles were slowing down or coming to a stop, backing up or entering or departing a parking space.

More than 4,300 pedestrians were killed in 2008, according to the most recent data available. The government has been researching the safety risks that hybrids and electrics could pose for pedestrians, particularly the blind, along with the elderly and children, for vehicles traveling at 20 mph or less. When a car is going faster, the friction between the tire and the road’s surface makes the vehicle louder.

The quiet hybrid phenomenon already has its place in pop culture. In an episode of NBC’s “The Office,” paper salesman Andy Bernard uses his stealthy blue Toyota Prius to sneak up on Dwight Schrute and pin his bitter rival against a hedge. One concerned co-worker, watching the unfolding drama, says “the Prius is silent if he keeps it under 5 miles per hour.”

Congress is heeding the warnings, adding sound performance requirements for hybrids and electric cars to an auto safety bill being considered after the massive Toyota recalls. Lawmakers could consider the changes this summer and car companies most likely would have to have the sounds ready to go three years after the release of new government rules.