In half a dozen states and many cities and counties, it is illegal to use a hand-held cell phone while driving -- but perfectly all right to talk on a hands-free device.

The theory is that it's distracting to hold a phone and drive with just one hand. But a large body of research now shoes that a hands-free phone poses no less danger than a hand-held one -- that the problem is not your hands but your brain.

Laboratory experiments using simulators, real-world road studies and accident statistics all tell the same story: Drivers talking on a cell phone are four times as likely to have an accident as drivers who are not. That's the same level of risk posed by a driver who is legally drunk.

Why cell phone use behind the wheel is so risky isn't entirely clear, but studies suggest several factors. No matter what the device, phone conversations appear to take a significant toll on attention and visual processing skills.

It may be that talking on the phone generates mental images that conflict with the spatial processing needed for safe driving. Eye-tracking studies show that while drivers continually look side to side, cell phone users tend to stare straight ahead.

They may also be distracted to the point that their engaged brains no longer process much of the information that falls on their retinas, which leads to slower reaction times and other driving problems.

At the University of Utah, driving simulators are used to study the effects of cell phone conversations. A simulator's interior looks like that of a Ford Crown Victoria, and a computer allows researchers to control driving conditions. Study participants are asked to drive under a variety of conditions: While talking on a hand-held phone or a hands-free one, while chatting with a friend in the next seat and even after consuming enough alcohol to make them legally drunk.

While in the simulator, drivers are asked to complete simple tasks, like driving for several miles along a highway and finding a particular exit or navigating local streets where they must brake for traffic lights, change lanes and watch for pedestrians.

How fast they drive, how well they stay in their lane, driving speed and eye movement are closely monitors.

The researchers have placed electrodes on participants' scalps to gauge how they process information. Similar studies, using brain imaging, have been done at Carnegie Melon.

The studies show that cell phone conversations are highly distracting compared with other speaking and listening activities in the car.

One might think that listening to talk radio or an audio book would degrade driving skill. It does not. A quiz after the driving test confirmed that the drivers were really paying attention to the programs.

Likewise, it is easy to equate talking to a friend on a cell phone with talking to a friend in the passenger seat. But a December report in the Journal of Experimental Psychology: Applied debunked that notion.

Utah researchers put 96 drivers in a simulator, instructing them to drive several miles down the road and pull off at a rest stop. Sometimes, the drivers were talking on a hands-free cell phone and sometimes, they were chatting with a friend in the next seat.

Nearly every driver with a passenger found the rest stop, in part because the passenger often acted as an extra set of eyes, alerting the driver to the approaching exit. But among those talking on a cell phone, half missed the exit.

Despite the overwhelming body of evidence that cell phone use while driving is risky, the idea of a total ban is sure to be controversial.