This really isn’t something that is open to debate. The studies have been done. Cell phone talk is far more distracting than talking to passengers. Here is one report excerpt:
“In fact, that “virtual” conversation is considerably more distracting than talking with a passenger, their study shows.
“Cell-phone conversations consume significantly more attention than passenger conversations, resulting in more incidents and crashes during simulated driving, Rose and Hunton concluded. “More working memory is consumed by cell-phone conversations relative to passenger conversations, and fewer resources are available for the driving task.”
The two professors specialize in studying the effects of technology on learning and awareness. Their article summarizing their research findings, “Cellular Telephones and Driving Performance: The Effects of Attentional Demands on Motor Vehicle Crash Risk,” appeared in the October 2005 issue of the journal Risk Analysis. No outside funding was received for the study, they note.”