The National Safety Center, which is not a government agency, published a White Paper on distracted driving in March 2010 that was the result of an extensive review of dozens of published studies on the subject. The paper concluded that driving while talking on a cell phone was as cognitively distracting as being legally intoxicated. There was no difference in the amount of distraction while using a hands-free device.
Texting while driving was at least four times more cognitively distracting. The difference between talking on a cell phone and talking to a passenger is that the passenger can see the same things you see on the highway, and act as a second set of eyes.
William L. Rigot