Way back, Jean Lindamood wrote for Car and Driver when cell phones were first coming into widespread use.
She wrote a great Op-Ed piece, concerning her own behaviour once she got a cell phone. Her main point was that with a cell phone in her hand, she was behaving very much like a drunk driver - following too close, and visually and mentally locking onto the vehicle in front, to the exclusion of everything else.
Remember, this comment is coming from a lady with advanced driver training, and who was paid handsomely to assess high performance vehicles.
Pretty much everybody's driving performance changes if they are using a phone in the car. And recently, the feedback seems to be that hands-free cell phone use does not correct the problem as much as many people think.
IMO, what is happening is that people's brains dedicate more bandwidth (I'm not being sarcastic) to processing conversations when somebody is not physically present. In contrast, your brain picks up all sorts of visual cues with in-car conversations, to some extent even if somebody is in the back seat.
Personally, even with hands-free, I keep my in-car cell use to an absolute minimum and primarily will pull off in a safe area to have a conversation.
That there have been many recorded incidents of fatalities with people texting sums up that option all too well.
Then there are the guys who mod their cars by taking out the driver air-bag to install a video display.
Must be our DNA segment that is tied to lemmings.
Be safe. Rgds, D.