[Gunner]

Any number of studies, specific to drivers and cel phones and otherwise (e.g., with people on intercoms) have shown that there's a significant cognitive difference between a conversation with someone present in the room and at least partially within the field of view, and conversations via an electronic link. Some theorize that the lack of personal presence forces the brain to use more effort to visualize the missing cues and interpret things from sound alone. In any case, the link between even hands-free cel phone use and an increased rate of driver distraction and accidents - even OVER anything attributable to passenger distractions - is well established and not trivial.