Hybrid cars are good for the environment. But scientists say that they might be bad for pedestrians. Because hybrids are so quiet, it can be hard for walkers to hear them until it’s too late. Listen to this. [quiet car sound]

That’s a Toyota Prius moving at five miles an hour. If you didn’t hear anything, well, that’s what worries scientists at the University of California, Riverside. They recorded the sounds made by an approaching hybrid vehicle and by a car with a standard, internal combustion engine. They then asked listeners wearing headphones to determine whether the car was coming from the left or the right.

Turns out people could hear the standard vehicle from about 36 feet away. But the hybrid was able to sneak up to just 11 feet away before listeners knew where it was coming from. And when the scientists added some background noise, people couldn’t pinpoint the direction of the Prius until it had already passed them by, results they’ll be presenting at the May meeting of the Acoustical Society of America in Portland, Oregon. The solution, they say, is to require a “minimum sound” for all late model cars. No annoying beeps or alarms. Just something to supply that new-car sound.

—Karen Hopkin