Enjoy (or fear) the silence while it lasts. Battery-driven vehicles are touted for their potential to cut down on harmful emissions spewed for decades by gasoline-powered cars, but electric and hybrid vehicles may be too quiet to be heard by pedestrians, posing a particular danger to people without sight.

Although the relatively low number of electric and hybrid vehicles on the road as well as the lack of data to link pedestrian injuries to quiet cars make it difficult to validate these concerns, one option being floated is that of "car tones" to make up for the missing engine noise, reports The New York Times. The Times cites Nissan, Toyota, BMW and plug-in hybrid maker Fisker Automotive as companies all considering the addition of sounds that would more prominently announce the presence of their cars on the road. (Stories of Fisker's plans to use speakers to pump out sounds "like something between a Formula One car and a jet plane" surfaced in March 2008.) "One possibility is choosing your own noise," BMW told the Times.

Congress, already considering the possibility that too-quiet cars could be dangerous to pedestrians, has versions of The Pedestrian Safety Enhancement Act of 2009 in the Senate and House that would direct the Transportation Secretary to study and establish a motor vehicle safety standard that provides for a means of alerting blind and other pedestrians of motor vehicle operation.

The most persuasive argument in favor of noisier electric and hybrid vehicles came last year via a study led by University of California, Riverside, perceptual psychologist Lawrence Rosenblum, who asked blindfolded subjects to listen to recordings of cars approaching at five miles per hour. As reported in the August 2008 issue of Scientific American, subjects could hear the hum of a Honda Accord's internal-combustion engine 36 feet away. But they failed to identify a Prius, running in electric mode, until it came within 11 feet—affording them less than two seconds to react before the vehicle reached their position.

The noise added to hybrids wouldn't have to be particularly loud, Rosenblum said, given the human brain's extreme sensitivity to approaching sounds relative to those that are fixed or moving away. Because the former are more likely to pose a threat, approaching sounds most readily stimulate regions of the brain associated with motor action. Whether it's a good idea to give drivers a selection of sounds to choose from is an open question. Shouldn't a car sound like…a car?

Image of Fisker Karmen plug-in hybrid © Fisker Automotive, Inc.