I believe that one factor that could make it dangerous even to use a hands-free phone while driving is the quality of the signal, that is, the signal-to-noise ratio of the auditory signal. The human mind is very good at separating signal from noise, and does it even without us noticing unless conditions are particularly bad, but it takes a great deal of cognitive effort, or what psychologists call "attentional capacity".

In addition to the audio being filtered to the audio band and sometimes being quite noisy, along with traffic noise, two other features of phone conversations make things even worse: hands-free setups often are monaural, which eliminates helpful redundancy, and since it is an auditory-only channel, all kinds of cues, primarily visual, are missing; this eliminates still more useful redundancy.

Of course, people differ in terms of how much attentional capacity they have (this is negatively impacted by some obvious short-term factors such as fatigue and long-term ones like aging), so there will still be individual or time-dependent differences.

Greg Shenaut

Good insights. They may also help explain how a hands-free cell phone conversation could be worse than a conversation with a passenger (although there may be other complicating factors like the tendency to turn toward or look at a passenger while talking).

The brain working to sort out a conversation with no body language input is certainly doable, but if I had to guess I'd say that's what causes the 1000-yd stare while you're on a cell phone and driving. When I'm conversing with someone actually in the car, I am watching them talk with their hands out of the corner of my eye, but we are also taking in the same scenery around us, and my focus is much closer to the action around my car.

FWIW, too, I can ALWAYS tell when one of the execs in my department is hurtling down a highway in a rented car talking on his Blackberry in a conference call, because he is NOT fully present for the conversations. There's a lot of "whats?" and repeats and confusion etc. Why call in at all, if I just have to type you a memo about the gist of the meeting so you can read it in your BVDs in the hotel? What a waste of everyone's time.

Maybe I wouldn't understand, though, because I never talk on the phone while driving (I hate talking on the phone anyway).

I've had cabdrivers on their cellphones when I got in the cab take me to the wrong destination and miss the turn for my apartment.

It's understandable that somone would want to make or take a call that someone will be late, but I think the calls that are the problem are the long drawn out ones. When I'm in a car I can always spot the people on cellphones because they drive slowly, have slower reaction time and drift all over the place.