Mobile phone use, while driving, slows down vehicular speed by about 2 miles per hour, according to a new study released by David Strayer, a psychology professor at the University of Utah.

"The distracted driver tends to drive slower and have delayed reactions," Strayer found. "People kind of get stuck behind that person and it makes everyone pay the price of that distracted driver."

Strayer based his conclusion after observing three dozen students driving simulators. Motorists using mobile phones are more likely to tail gate a slow car ahead and change lanes 20 percent less than drivers not using their cellphones.

On a gridlocked route, it will take a driver using his mobile phone 3 percent longer to finish his journey and 2 percent longer on a medium-congested avenue.

Mobile using drivers add up 20 hours a year to an American commuter's trip, Strayer's study said.

The psychology professor explained that the slowdown happens due to brain overload. The brain's frontal cortex can process only so many tasks at a given time, that the normal response is to slow down.

"We found that when people are on the phone, the amount of information they are taking in is significantly reduced... People were missing things, like cars swerving in front or sudden lane changes. We had at least three rear-end collisions," Strayer said.

Annie McCartt, senior vice president of the Insurance Institute for Highway Safety said the study generally makes sense, but simulator results are different from real world conditions.

Strayer's report will be presented later this month to the Transportation Research Board of the National Academy of Sciences.