Talking about cognitive load

Cognitive Daily reviews a study looking at driving while talking on the phone or talking to a passenger.

The task was to hold a conversation while driving down a busy, simulated highway, with the goal of taking an exit eight miles away.

Fifty percent of the drivers who were talking on the cell phone missed the exit, while only 13 percent of the drivers talking to the passengers did….

What’s more…in conversations with passengers, the discussion shifted to the traffic / driving situation nearly twice as often than in conversations on the phone.

I can't talk right now...I’ve been reading about the social aspects of communication — e.g., that it’s not just the message, but the relationship between the parties involved. Even granting the limitations that the Cognitive Daily post points out, I wonder whether we don’t have a different (implicit) mental model for a conversation with someone who’s physically present from the one we use in a conversation with someone who’s virtually present (on the phone).

I’d like to know about studies looking at interruption. My hunch is that people will stop a face-to-face conversation to pick up the phone more than they’ll stop a phone conversation to talk to someone who’s just shown up.