The differences are apparent at the operational, tactical and strategic levels of performance, the researchers state. The study, which was done in a sophisticated driving simulator, not on the street, analyzed the driving performance of 41 mostly young adult drivers paired with 41 friends who served as conversation partners. Both sexes were equally represented.

In each of three experimental conditions (conversation with hands-free cell phone, conversation in the car, or no conversation), one person in each pair was randomly selected to be the "driver" and the other the conversation partner.

Drivers and partners spent 10 minutes on a 24-mile multilane highway with onramps and offramps, overpasses and two-lane traffic in each direction. Participants drove under an irregular-flow condition that mimics real highway conditions, with other vehicles, in compliance with traffic laws, changing lanes and speeds. This context required "drivers" to pay attention to surrounding traffic. Partners were told to tell each other a previously undisclosed "close call" story about a time their lives were threatened.

Results show the cell-phone users were more likely to drift past their lane boundaries, kept increasing following distance and were four times more likely to miss pulling off the highway at the designated offramp. Passenger conversation barely affected all three measures.

Problems could have stemmed from inattention "blindness" or insufficient processing of information from the driving environment. Cell-phone users may also find it more difficult to hold the thought to exit at the designated ramp.

The more complicated the task, the more drivers appeared to modulate the complexity of their speech, as measured by syllables per word. Drivers also talked more when using cell phones, the authors suggest, because perhaps they were trying to control the conversation to avoid using the mental resources required to really listen to the other person.

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