Nissan Leaf (© Nissan North America)Click to enlarge picture

While it was widely rumored that Nissan's electric Leaf would be given futuristic sound effects as a safety measure, it is more likely that the company will be responding to forthcoming directives from the National Highway Traffic Safety Administration.

A flood of articles and blog posts swirled around Nissan last year after Bloomberg.com reported that the automaker planned to give its Leaf electric vehicle a space-age "whirring" sound like the flying cars in the film "Blade Runner."

The reason being that electric vehicles, and hybrids that can run in electric-only mode at low speeds, are increasingly being seen as a risk to pedestrians, cyclists and especially the blind. Why? When rolling down the road, these vehicles are as quiet as church mice. Consequently, momentum is building among legislators to require that EVs and hybrids alert pedestrians to their presence. Currently there are no laws that force automakers to add sound effects to their cars, but this sort of regulation is very likely on its way.

In the meantime, every major company that sells hybrids, or has ambitions to sell EVs or plug-in hybrids, is already working on the issue behind the scenes.

The Issue
Above about 20 mph, the noise of air turbulence and a car's tires on the road make even virtually silent cars audible. But many hybrid models, such as the Toyota Prius and Ford Fusion Hybrid, can drive at low speeds on just battery power. It's while moving at these low speeds, while maneuvering around parking lots, driveways and intersections, that hybrids and EVs are thought to pose the biggest threat.

Only a few studies have examined whether hybrids and EVs are more dangerous to pedestrians, but the results so far suggest that they are.

In September 2009, the National Highway Traffic Safety Administration published a study comparing the accident records of hybrids and conventional internal-combustion cars in 12 states. When moving in a straight line, hybrids showed no greater incidence of colliding with pedestrians or bicycles. But when slowing down or stopping, backing up, or getting in or out of a parking space, hybrids were twice as likely to hit a pedestrian. Hybrids were also more likely to collide with a cyclist at an intersection or interchange.

Compare: Toyota Prius vs. Honda Insight vs. Ford Fusion Hybrid

A separate study looked at the distance at which a pedestrian can detect the sound of a conventional car versus a hybrid in electric-only mode. Lawrence Rosenblum, a psychologist specializing in everyday perception, recorded the sound of hybrids and normal cars at his lab at the University of California, Riverside. Rosenblum then played the recordings for test subjects in his lab.

When in electric-only mode, a hybrid needed to be 40 percent closer before test subjects could indicate whether it was coming from the right or left. Only when the sound of a hybrid was 7 feet away, or about a second from impact, could the subjects determine the oncoming direction.

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Disturbing, But Irrefutable?
The research is telling, but a long way from conclusive, says Thomas Mutchler, senior automotive engineer for Consumers Union, the advocacy group that publishes Consumer Reports. "Right now it looks like hybrids do pose more of a risk than typical gasoline-powered cars, but the level of increase of that risk isn't quite certain," he says.

In Mutchler's view, the NHTSA study suffers from a relatively small sample set (only a few thousand documented accidents) and doesn't take into account accidents per mile driven. He does, however, acknowledge, "It does point to a trend of hybrid cars posing a greater risk."

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Not everyone agrees. Daryl Siry is the former vice president of sales and marketing for Tesla Motors, the maker of the all-electric Tesla Roadster. He says the difference between an electric car and the quiet purr of a nonhybrid Honda Accord moving at parking lot speeds is negligible: "It's kind of funny that people would raise this [issue]. Seems overblown to me." If electric cars are required to emit a noise, Siry says it should be customizable and something the driver can turn on and off at will.

The National Federation of the Blind says the issue could be a matter of life and death. It has been aggressively pushing for legislation that would require hybrids and electric cars to emit a recognizable sound at low speeds.

What do you think: Can an electric vehicle's inherent silence be deadly?

NFB President Mark Maurer says, "The one reliable clue that gives us [those without sight] the information we need to navigate safely is not available to us when a silent car is involved. For us, these cars are invisible. Because they are invisible, catastrophe is inevitable."