The study analyzed the driving performance of 41 young men and women drivers paired with 41 friends who served as conversation partners. The conversation partners were told to tell one another a previously undisclosed "close call" story about a time their lives were threatened. Some drivers used a hands-free device, others had a passenger sitting next to them. Even when drivers used a hands-free cell phone, driving performance was significantly compromised.

"Cell phone and passenger conversation differ in their impact on a driver's performance; these differences are apparent at the operational, tactical, and strategic levels of performance," the researchers wrote in the Journal of Experimental Psychology: Applied.

Drivers used a driving simulator of a 24-mile multilane highway with on- and off-ramps, overpasses and two-lane traffic in each direction. Participants drove under conditions that mimic 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.

The drivers using cell phones drove significantly worse than those talking to passengers. The cell phone users were more likely to drift in their lane, kept a greater distance between their car and the car in front, and were four times more likely to miss pulling off the highway at the rest area. Passenger conversation barely affected all three measures.

Previous studies by Strayer and Drews found that hands-free cell phones are just as distracting as handheld because conversation is the biggest distraction. They also have shown that when young adults talk on cell phones while driving, their reaction times become as slow as reaction times for senior citizens, and that drivers talking on cell phones are as impaired as drivers with the 0.08 percent blood alcohol level--the level that defines drunken driving in most states.

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