New York has become the first state to take the official position that cell phones and driving don't mix. Today the state assembly passed the nation's first state ban on the use of hand-held cell phones while driving. The state senate has already approved the bill, and the Governor has said he'll sign it.

Let's take this New York story one step further and talk about hands-free use of cell phones. You build in a system, or just attach the headphone and mouth piece and talk. David Strayer has been researching the cell phone safety issue, for an article that's to be published in the journal Psychological Science later this year. He says, even if you use a hands-free modal, it's still difficult to talk on the phone and drive.

More often than not it's just the fact that you're paying attention to the conversation, the semantics of it, what is being said, and usually those conversations don't deal with the driving environment. So paying attention to that conversation makes it difficult to attend to things that are related to the specific diving situation you are in.

What if you're sitting next to, your wife is in the car and you're having a little, let's say argument. Certainly there you're caught up in the discussion, but I would gather you wouldn't think that's so dangerous?

Well, it may not be the best of driving circumstances, that's for sure. But there are cases where conversations have been linked to accidents. But, normally, when you have two people in a car they're aware of the driving circumstances and they typically adjust their conversations. As soon as you get on a cell phone conversation they aren't aware of the moment to moment driving conditions so they can't modulate their conversations according to the driving difficulty.

Now, you've been doing some testing that would support some of your theories.

Well, we've done studies where we've had people carrying on conversations while trying to react to traffic lights, red and green lights. When they see a red light, they need to just hit the breaks as quickly as they can. What we found is that, when you're talking on a cell phone, either a hands-free or handheld cell phone you're twice as likely to miss those traffic signals when they're presented.

Now what about the radio; what about playing books on tape, that kind of thing?

We tested that. We kind of did a heads up comparison with listening to a radio or listening to segments of Tom Brokaw's book on tape The Greatest Generation. Neither case produced any interference. So, it seems to be something about that active engagement in the conversation that takes you away from the immediate here and now into this kind of a virtual reality that's created by the cell phone conversation.

What do you think is really going on here? I have noticed that when I try to do a talk on the phone, while driving, that I sort of go into a trance and realize that I've missed exits and it's almost like I've been asleep in a way.

People for a very long time, over 100 years, have known that it's very difficult to do two things at once that they really, very well, if they require a tremendous amount of attention. You've gotta pay attention to things. You'll find that if I pay attention to one, other things that I need to pay attention to will not be processed as well. That is what we think is going on, is that, especially with respect to driving; what we see is that, we have limits in what we can attend to.

So, you're saying, if you're talking on a telephone, whether you're holding or not, in a certain situation, somebody jumps out in front of you, you could be in trouble.

That's right. We did studies where we actually asked, "What is it that is vision impaired while you're talking on a cell phone?" We had people look at objects that were presented right where they were looking while they were driving, and we said, "Did they see them as well when they're talking on the phone as when they weren't?" And they didn't. Even though things are falling right on your eye, if you're not attending to that information, it's just not processed.

One of my colleagues at Harvard, Dan Simon's, been kind of showing that really well in a non-cell phone conversation. He had two teams playing kinda catch with a basketball and, the subjects were told, "Pay attention to the team wearing the white jerseys." And then right in the middle of that video clip, a guerrilla walks out, beats its chest right in front of, right in full view and walks away.

Then Dan asked, "What did people see?" Half the people didn't see the guerrilla, even though this isn't some subtle event. It's a big gorilla that walks out stands there, beats its chest and walks off.

What you attend to determines what you perceive, and if you're not attending to the driving environment, or you're not attending to the guerrilla, then you just don't see it.

David Strayer, Associate Professor of Psychology at the University of Utah, speaking with us from Salt Lake City.