Hybrid cars are so quiet when operating only with their electric motors
that they may pose a risk to the blind and some other pedestrians,
research by a University of California, Riverside psychologist
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
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,
"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
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
"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