Talking while driving -- even using a hands-free kit -- makes motorists four times more likely to have an accident, according to David Strayer, a University of Utah psychology professor who led the study, which tracked drivers’ brain activity.

Strayer said chatting on a cell phone while driving cuts the brain’s ability to operate a vehicle and process information about traffic by 50 percent.

The study asked 16 pairs of friends to "drive" using a highway-driving simulator while chatting on the phone. Meanwhile, electrodes measured the brain’s activity and eye movements as a variety of situations cropped up -- from a car braking suddenly to a vehicle abruptly changing lanes and disrupting the flow of traffic, he told AFP.

In each instance, the brain’s ability to react was cut in half.

Strayer noted, however, that a telephone conversation does not have the same impact as a conversation with an adult passenger, because in the latter case, the driver "has an interest in driving safely. He can say ‘I should stop talking.’"

Cell phones have been outlawed for motorists in only a half-dozen US states, according to the researcher.