SIOUX FALLS, S.D. (AP) — The silent approach of a hybrid car could have catastrophic results for pedestrians who can't hear them, which is why blind people are working with the auto industry to research the idea of adding sounds to the otherwise quiet new cars.

"They not only can be dangerous for blind people, they can be dangerous for sighted people as well," said Bob Riibe, a blind person who relies on the sound of traffic to know when it is safe to walk across streets in Sioux Falls. "When that motor is going, you just don't hear it."

A hybrid runs quietly when it is powered by electricity at lower speeds on city streets. At higher speeds, such as on a highway, it relies on a combustion engine.

Their numbers are on the rise, and Julaine Arient-Rollman of Rapid City fears that she and other blind people could lose some independence if they are no longer able to safely cross streets.

"If they continue with quiet cars, they're going to isolate us," she said.

Riibe — who originally is from Sioux City, Iowa, and now lives in Sioux Falls — is vice president of the South Dakota chapter of the National Federation of the Blind. The federation is working with automakers to find a solution.

Congress also is getting involved in the issue. The Pedestrian Safety Enhancement Act of 2009 would require a two-year study exploring possible solutions, and regulations put in place based on the study's findings. Rep. Stephanie Herseth Sandlin, D-S.D., is among several co-sponsors in the House.

"We feel like there is broad recognition now that this is an issue that needs to be addressed," said Chris Danielsen, a spokesman for the Baltimore-based National Federation of the Blind.

Members of the group are optimistic about the bill's chances.

"I think right now there's a lot of optimism that the bill is going to move forward," said Jesse Hartle, government program specialist with the federation. "Some members of the auto industry are recognizing that this is becoming more of a safety issue."

Automakers have been doing intense research to figure out what sounds are helpful to blind pedestrians, among other questions, said Wade Newton, a spokesman for the Alliance of Automobile Manufacturers, an industry association of 11 automakers including Toyota, General Motors and Ford Motor Co. Researchers also are trying to learn more about how vehicles produce the noises people hear, such as the sound of a car piercing the wind at higher speeds.

"Certainly, it's a concern of ours," Newton said. "Any time a group of consumers expresses concerns with our products, it's important to us."

But figuring out how to add the right noise to a vehicle presents several challenges, and a balance must be found between the concerns of pedestrians and those of the community at large, Newton said.

"Do you design a sound that can be heard above the hustle and bustle of Times Square?" he asked. If so, would that sound be acceptable after the car leaves Manhattan and enters a quiet neighborhood?

"I think there's some thought out there that there needs to be uniformity, and a consistent sound," Newton said. "What if different companies choose different sounds?"

In other words, what if an approaching Honda sounds like a bird chirping, but a Toyota's warning comes in the form of musical notes?

"I don't have a preference," Riibe said. "I just want a noise that's going to be loud enough to be heard in traffic just like the combustion engine is now."