Cell phones have been labeled dangerous in a handful of studies cataloguing how they distract drivers. One solution in many minds is the "hands-free" phone.

But it turns out that's a lousy idea, too.

In fact, your reaction time behind the wheel seems to be slowed whenever you are doing anything but just driving.

Pushing buttons

In a new study, 40 students drove a car simulator with a steering wheel, gas and brake pedals and a large plasma screen. They followed a lead car and were told to brake as soon as they saw brake lights.

They were also asked to do other simple tasks, such as press a button on the steering wheel or say a word out loud when they saw a light flash in the lead car's rear window.

On average they were 174 milliseconds slower at braking when the two tasks occurred at the same time than they were when the tasks were presented 350 milliseconds apart.

That 174-millisecond delay translates to 16 feet in a car going 65 mph, the scientists say.

We can't look and listen

Importantly, the delays were the same whether the tasks involved visual or audible input, vocal or manual responses.

"This study joins a growing body of research showing that 'freeing up the hands' does not result in faster brake response times," said Jonathan Levy of the University of California, San Diego.

The reason, according to a study last year, is that the human brain struggles to look and listen at the same time.

The new results are detailed in the March issue of the journal Psychological Science.

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