Cell phones clog traffic

If you commute by car an hour a day, it could all add around 20 hours a year to your commute, Strayer said.

"The distracted driver tends to drive slower and have delayed reactions," said Strayer, whose study will be presented later this month to the Transportation Research Board of the National Academy of Sciences. "People kind of get stuck behind that person and it makes everyone pay the price of that distracted driver."

Strayer's study, based on three dozen students driving in simulators, found that drivers on cell phones are far more likely to stick behind a slow car in front of them and change lanes about 20 percent less often than drivers not on the phone.

Overall, cell phone drivers took about 3 percent longer to drive the same highly traffic-clogged route (and about 2 percent longer to drive a medium congested route) than people who were not on the phone. About one in 10 drivers is on the phone so it really adds up, said Strayer, whose earlier studies have found slower reaction times from drivers on the phones and compared those reaction times to people legally drunk.

Combine those factors and Strayer figures distracted drivers are adding an extra 5 to 10 percent of time to your commute.

It's simply a matter of brain overload. Your frontal cortex can handle only so many tasks at one time, so you slow down, Strayer said.