Hybrid cars have been hailed as the answer to a litany of problems in the U.S., from global warming to gas prices. But now one group is labeling them as something else: a threat.

According to the National Federation of the Blind, hybrid cars, which run on an energy efficient mix of gasoline and electricity, are problematic for children, elderly pedestrians and the blind.

Why? Hybrids are much quieter than their conventional counterparts, so silent that the NFB says that they can be difficult to hear coming down the street. The group conducted tests to try to prove their point.

"We had [a hybrid] drive by in different situations, to see whether or not people could hear it and use the sound of the car to safely cross the street, and they could not. The car was just silent," NFB spokesman John Pare told AFP.

In order to make the cars more audible and safer, the NFB would like to see manufacturers of the vehicles emit a signal or sound that would alert pedestrians and others of an oncoming car.

"We want to get ahead of this and not have to wait until five blind people end up seriously hurt or dead," Gary Wunder, who is on the NFB's Committee on Automobile and Pedestrian Safety, told the Wall Street Journal.

The group says that it has contacted hybrid manufacturers about making the adjustments. One such automaker, Toyota (which makes the popular hybrid the Prius), told AFP that they are looking into ways to improve pedestrian safety, but that the general trend in the auto industry is towards quieter cars and that the silence of the cars has benefits, too.

"We are aware of this issue and are studying a way to improve pedestrian safety. It's important that the solution doesn't add to noise pollution but is good for pedestrians," said Toyota spokeswoman Martha Voss.

In any case, some groups are taking action to help prevent accidents before they happen. According the Wall Street Journal some guide dogs, which use car noise and other signs to help the blind cross streets safely, are being trained to be more aware of the quiet cars. One guide dog school on the West Coast is even using golf carts to simulate the hum of the hybrid engine.

FOX News' Alexander B. Duncan contributed to this report.