The dangers of distracted driving have been in the news lately after a horrific Christmas Eve car accident where a New Hampshire firefighter pulled his dying daughter from the scene of the wreckage. Police say the driver who killed the mother of three was likely driving while distracted, perhaps by an incoming call.

In fact, a study published this week in the New England Journal of Medicine found that we spend 10 percent of our time behind the wheel in a distracted state—putting on makeup, fiddling with the radio station, or handling our phones. And this increases the risk of getting into accidents, especially among teen drivers who are novices.

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The researchers placed monitoring devices—accelerometers, cameras, and other sensors—in the cars of 42 teen drivers and 109 experienced adult drivers for up to two years to determine the circumstances behind 685 crashes and near-crashes that the drivers were involved in.

For teen drivers, dialing a phone while driving led to an eight times higher crash risk. Reaching for a phone increased the teen crash risk by seven times, and sending or receiving text messages increased the risk four-fold. Other things that increased crash risk: reaching for an object other than a phone—like a soda can—or rubbernecking to look at a roadside accident.

Talking on a hands-free device didn’t appear to increase the risk of crashing, but the researchers said it appeared to be somewhat distracting and degraded driving ability in teens.

Among experienced adult drivers, dialing a cell phone led to a 2.5 times greater risk of crashing. Texting and browsing the web probably increase crash risk as well, but these two phone activities weren’t assessed in the adult driver study, which was conducted nearly a decade ago by the researchers—before smart phones came into widespread use.

“Our research and previous studies have shown that taking your eyes off of the road for longer than two seconds—for any reason—significantly increases your risk of having a crash,” said study co-author Bruce Simons-Morton, a senior investigator at the National Institute of Child Health and Human Development which provided funding for the study. “Virtually everything involving the use of a smart phone for teens was risky except for talking on the phone itself.”

Despite those results, Simons-Morton said state laws in Massachusetts and elsewhere banning all cell phone use for teen drivers make sense. “The culture among youth is to immediately respond to text with a text,” he said. “They bring this habit into driving when they’re just learning how to operate a car, which makes it particularly risky.”

Parents need to emphasize that phones need to be turned off when their teen is driving because the temptation to answer a text or reach for a ringing phone will likely be too great to avoid.