Originally Posted by caloso
PaperBoy, if I recall correctly, it's that when you talk on the phone your brain needs to create an image of the person on the other end of the line. This uses up brain resources. You don't do that when you talk to the person in the next seat because you can just look at them.
I think it has a lot to do with the fact that there are other communications going on between folks situated within physical proximity of each other besides voice and hearing.
For one thing if you have to slam on the brakes for something the passenger can see and feel what is going on, someone on the phone cannot, and you know that. It's easier to communicate effectively with someone who is right there with you, and, hence, requires less concentration and focus.