When it comes to hybrid cars, silence is not so blessed. Human factors researchers at North Carolina State University say the vehicles are too quiet for safety – pedestrians and bicyclists can't hear them coming. The researchers want hybrid manufacturers to build in some noise, and recommend automotive engine sounds.
 
In their paper published in the Proceedings of the Human Factors and Ergonomics Society, 52nd Annual Meeting, Patrick Nyeste and Michael S. Wogalter evaluated the responses of 24 participants (mean age = 19.4 years) to six categories of noise: engine, horn, hum, siren, whistle and white noise. Three variations of each type of sound were tested.

By a wide margin, the participants preferred automotive engine sounds. It is noise associated with a familiar danger. White noise was ranked second, with a hum in third place. 

A news release about the findings, published recently by the Human Factors and Ergonomics Society (HFES), notes that automakers have continually worked to make passenger vehicles quieter, and now find themselves with a need to make their quietest vehicles louder. Noise pollution could be avoided, if a “smart system” alters the level of emitted sound depending on the levels of vehicle and background environmental sound. These systems would turn themselves off if the vehicle produces adequate sound on its own.
 
Lotus Engineering is addressing the quiet hybrid issue, according to the HFES news release, with its "Safe and Sound" system, which mimics the sound of an internal combustion engine and operates when the vehicle is in electric-only mode.

The researchers note that their findings also apply to other silent vehicles. Electric golf carts, bicycles, wheelchairs and Segways have caused injuries because they are quiet.
 
The National Federation for the Blind has called for quiet vehicles to emit a continuous sound and for additional research on the subject.

The National Highway Traffic Safety Administration, automobile manufacturers and the Society of Automotive Engineers International are exploring solutions to the problem. The Pedestrian Safety Enhancement Act of 2008, which would require the Secretary of Transportation to study and implement regulations for hybrid, electric, and other silent-engine vehicles to emit nonvisual alerts for pedestrians, is under consideration in the United States Congress.

Source: Human Factors and Ergonomics Society