Reuters reports that a US study has mobile phone calls distract drivers far more than even the chattiest passenger, causing drivers to follow too closely and miss exits.
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.
It demonstrated that chatting on a cell phone can slow the reaction times of young adult drivers to levels seen among senior citizens (who at least drive more slowly), and showed that drivers using mobile phones are as impaired as drivers who are legally drunk.