Driving while distracted

We all know driving is a complicated business, but that doesn't keep us from doing a lot of other things behind the wheel.

So when we siphon off some brainpower to talk on our cell phones, how much does it really matter?

A lot, say Boston researchers. As in, reducing emergency stopping distance by more than a car length if you are driving 60 miles an hour.

People tracking multiple objects on a screen -- a simulation analogous to the visual, physical and cognitive demands of driving -- took a little longer to react when they were just listening to someone talk, researchers reported. But listening and talking on a cell phone added significantly to their reaction times.

"If we assume that this result would generalise to driving, talking on a mobile phone would lead a driver going 60 mph to travel an additional 18.5 feet (more than the length of an average car) before braking," Todd Horowitz and Michael Cohen of the Brigham and Women's Hospital and co-authors write in the Psychonomic Bulletin and Review.

Don't think your hands-free phone gets you off the hook. The researchers say putting conversation on a speaker phone made little difference. It's the conversational give-and-take that drains some of the attention needed to drive, adding to competing demands on our visual and cognitive abilities.