Drivers talking on cellphones are probably making your commute even longer, concludes a new study.

Motorists yakking away, even with handsfree devices, crawl about three kilometres per hour slower on commuter-clogged roads than people not on the phone, and they just don't keep up with the flow of traffic, said study author David Strayer, a psychology professor at the University of Utah.

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 cellphones are far more likely to stick behind a slow car in front of them and change lanes about 20 per cent less often than drivers not on the phone.

Overall, cellphone drivers took about three per cent longer to drive the same highly traffic-clogged route (and about two per cent 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 five to 10 per cent 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.

Generally the study makes sense, but what happens to students in a simulator may not translate to real world conditions, said Anne McCartt, senior vice-president of the Insurance Institute for Highway Safety. Further, she said the study itself points out how distracted drivers are slower, but is short on calculations on just how it affects other drivers.

Wireless phone companies encourage people not to talk on the phone in bad traffic, said Joe Farren, a spokesman for the cellular phone industry's trade association. But he said he couldn't comment on the study because he had not had a chance to go over it.