In some contexts, pedestrians may have only one second to audibly detect the location of approaching hybrid cars when the vehicles operate at very slow speeds, said Lawrence Rosenblum, professor of psychology. Those findings have implications for pedestrians who are blind, small children, the elderly, runners, cyclists, and others, he said.

“There is a real difference between the audibility of hybrid vehicles and those with traditional internal combustion engines that could have effects on the safety of pedestrians, which needs to be studied,” Rosenblum said. “Our findings could mean that there is an added danger with hybrid cars, particularly at intersections and in parking lots.”

In an on-going research project funded by the National Federation of the Blind, Rosenblum made audio recordings of hybrid and combustion-engine cars approaching from two directions at 5 miles per hour to assure that the hybrid car operated only with its electric motor. Subjects in a lab listened to the recordings and indicated when they could hear from which direction the cars approached.

In one study, the background sounds of two quietly idling combustion-engine cars were added to simulate the noise of a parking lot. With these stimuli, the hybrid needed to be 74 percent closer than the combustion-engine car before the subjects could hear from which direction the cars approached.

“Subjects could correctly judge the approach of the combustion car when it was about 28 feet away,” Rosenblum said. “But they could only judge the hybrid’s approach direction when it was seven feet away.” This means that a pedestrian would not be able to correctly determine the hybrid’s approach until it was one second away, he said.

Preliminary findings released in March found that without the addition of background sounds the hybrid car needed to be about 40 percent closer than the combustion-engine car before the subjects could determine whether it was approaching from the left or right.

At speeds above 20 to 25 miles per hour hybrid cars likely generate enough tire and aerodynamic noise to make them sufficiently audible, Rosenblum said.

“This research provides evidence that hybrid cars, when operating in silent mode, pose a substantial risk to blind people and other pedestrians. We hope that regulators and car manufacturers will take notice of these results and take steps to eliminate this risk,” said Dr. Marc Maurer, president of the National Federation of the Blind, a 50,000-member advocacy organization for people who are blind or have low vision.

Rosenblum, who is an adviser to the Society of Automotive Engineers, has spent many years researching perception of approaching cars and whether there are similarities between visual and auditory perception of approach.

“I really do feel this is an issue for more than those who are blind,” he said. “We’re also talking about bike riders, runners and others. Walking around with my kids in a parking lot makes it very clear that I’m using hearing and vision to determine where things are.”

Rosenblum eventually will test people who are blind in parking lots to determine the level of risk. He met recently with Stanford University researchers who are developing different sounds that would enhance the ability of pedestrians to hear approaching hybrid and electric cars.

“Everyone’s aware of the issue,” he said. However, Rosenblum said, “We are not talking about major changes to the way automobiles are designed, but about slightly increasing their audibility when they are traveling slowly. Only a subtle sound enhancement should be required — maybe something like the simulated sound of a very quiet engine.”

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