Mobile phone calls distract drivers far more than even the chattiest passenger, causing drivers to follow too closely and miss exits, US researchers reported.

Using a hands-free device does not make things better and the researchers believe they know why - passengers act as a second set of eyes, shutting up or sometimes even helping when they see the driver needs to make a manoeuvre.

The research, published in the Journal of Experimental Psychology: Applied, adds to a growing body of evidence that mobile phones can make driving dangerous.

Lee Strayer of the University of Utah and colleagues have found in a series of experiments using driving simulators that hands-free mobile phones are just as distracting as handheld models.

They have demonstrated that chatting on a cell phone can slow the reaction times of young adult drivers to levels seen among senior citizens, and shown that drivers using mobile telephones are as impaired as drivers who are legally drunk.

For the latest study, also using a simulator, Dr Strayer's team showed that drivers using a hands-free device drifted out of their lanes and missed exits more frequently than drivers talking to a passenger. They tested 96 adults aged 18 to 49.

"The passenger adds a second set of eyes, and helps the driver navigate and reminds them where to go," Dr Strayer said in a statement.

"When you take a look at the data, it turns out that a driver conversing with a passenger is not as impaired a driver talking on a cell phone," he added.