A new study has confirmed the widely-held belief that cellphone users pay less attention to the road. But it appears that it's concentrating on conversation, rather than holding the phone, that's distracting.

Researchers at the University of Rhode Island used a head-mounted eye tracking device to measure people's eye movements as they drove. They found that people talking on cellphones exhibited so-called tunnel vision, a restriction of side-to-side eye movements that prevents drivers from observing potential hazards on the side of the road.

"With the simulated cell phone ... we found that the range of eye movement stops significantly," says Professor Manbir Sodhi, who conducted the tests.

Sodhi and other researchers tested users of both hand-held and hands-free cellphones and found it's not holding the phone that's a problem, it's talking on it.

"The debate surrounding cell phone use in cars has been directed toward concerns over holding the phone," says Sodhi. "Holding the phone isn't the main issue. Thinking is."

The researchers found that the tunnel vision doesn't end as soon as the conversation does, perhaps because drivers are still thinking about the content of the call.

"It's really that the brain can only do so much at one time," says Professor Marcel Just of Carnegie Mellon University, where researchers measured brain activity while subjects drove down a simulated highway.

"There's a very substantial decrease in the amount of brain activity, the amount of neural activity allocated to driving while you are simultaneously listening," Just says.

Surprisingly, the researchers aren't calling for a ban on cellphone use while driving. Instead they say more effort should go into promoting safe cellphone use.

"I believe higher levels of safety can be achieved by establishing cell-free zones," Sodhi says.

He suggests that cell phone use be prohibited on roadways that require high levels of alertness, such as in congested traffic or poor weather conditions. On flat, dry, open pavement during light traffic conditions, he believes cell phone use should be permitted.