File this one under 'no-freakin'-duh':

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

Using a handsfree 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 maneuver…

…They have demonstrated that chatting on a mobile 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.

Check out the Reuters story or the original study.