That requires you to concentrate more on the conversation, and thus less on the driving, because you can't devote equal attention to both. That's true even if the person on the cell phone is simply trying to give you directions.
"I have to form a mental map of where I'm going and that sort of visual processing is going to be taking away from what I'm seeing out the windshield," he says.
Either reaction time or performance will be impaired, he adds.
I interviewed Yantis on the telephone, and while we were talking I noticed personal evidence of the validity of his research. When I concentrated on understanding what he was saying, my vision literally blurred. My eyes weren't focused on anything in my office, and everything was a bit fuzzy.
Try it some time.
"If you really want to concentrate close your eyes," he says, "because the visual scene will distract you a little bit."
Of course, not while driving a car.
Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.