[goblin89]

> One of the problems is that whatever you happen to be focusing on, you also have to be able to react quickly to potential hazards.

True. From my anecdotal observation the reason why talking over mobile phone is dangerous (while talking to a passenger is not) seems to be social norms—immediately cutting off the conversation to focus on the road doesn't seem like an acceptable thing to do, and we avoid doing this unless absolutely necessary. For a more responsible driver, this ‘absolutely necessary’ threshold is pretty low—as it should be—but such people seem to be rare.