Hybrid cars, such as the Toyota Prius and the Ford Fusion, are capable of using electric power instead of burning gasoline to operate. While a hybrid is using electric power, it is a lot quieter. This can create a safety hazard for pedestrians, bike riders, and skateboarders, who may not hear that a car is driving along the road. The federal government passed a law to make hybrid cars louder as part of the Pedestrian Safety Enhancement Act of 2010.

The main focus of the law is on helping blind pedestrians. A blind pedestrian can’t see a car coming and is used to hearing vehicles from some distance away. To test the safe distance, scientists measured the distance that a blindfolded pedestrian needed to hear a car coming. According to the National Institutes of Health, an experiment showed that the distance before the person could hear a regular gasoline powered car was 36 feet, and the distance before the person could hear the hybrid running on electricity was 11 feet.

The regulation does not specify the type or volume of noise that the hybrid car should make, other than that blind people should be able to hear it at a distance. A speaker could produce the sound of a normal car engine, which could be as loud as a regular vehicle. The sound system could also make a unique noise, because loud car engines already bother the residents of some areas and the driver might want another car effect for vanity purposes.

Some vehicle buyers purchase hybrid vehicles for their quietness, in addition to their reduced emissions and fuel efficiency. The National Park Service uses hybrid buses to carry tourists around Yosemite. The loud noise of a vehicle engine would upset the birds and other wildlife, and disrupt the vacations of other visitors. The sound volume in a remote area of Yosemite may be as much as 16 times less than the volume in a city.

A national park is much more quiet than a city because of natural selection, according to Carleton University. Animals use up energy by making noise. An animal usually makes noise only to send a message to another animal, such as showing off during a mating ritual or scaring away a predator, and is quiet the rest of the time.