Washington's new law that permits police to stop drivers for the sole reason of holding a cellphone does not solve the problem of distracted driving. Guest columnists Daniel Simons and Christopher Chabris argue it is the conversation that distracts the brain, not the cellphone itself.

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BY now we all should know that talking on a cellphone while driving is dangerous, and that texting while driving is absurdly distracting.

A new Washington state law taking effect next Thursday makes holding a phone while driving a primary offense. Unfortunately, the law is unlikely to have much impact on distracted driving because it is based on a common and fundamental misunderstanding of how human perception works.

Most people believe the distracting effect of cellphones can be eliminated by keeping your eyes on the road and by using a hands-free phone. However, the dangers of driving while talking on the phone come not from our hands or from our eyes, but from our brains.

Some years ago, we did a study that has become one of the better-known illustrations of just how easily we can miss the obvious when we are distracted. In our experiment, we asked subjects to watch a short video of two teams of people passing basketballs. The task was simple: Count how many times the team wearing white passed the ball, while ignoring the passes by the team wearing black.

Unbeknownst to the subjects, we weren’t really interested in how well they could count the passes. Rather, we were interested in whether or not they would notice when a person wearing a full-body gorilla suit unexpectedly entered the scene, walked to the center of the game, turned to face the camera, thumped its chest, and exited the scene on the left. Even though the gorilla was visible for 9 seconds, half of the subjects in our experiment didn’t notice it at all.

Although this failure of awareness is interesting in its own right, the more important finding from our study is the shock people expressed when shown what they had missed. Most of us fully expect that we would notice if something as remarkable as a gorilla entered our field of view. We intuitively believe that unexpected events will “capture” our attention, even if we are actively doing something else.

Yet studies of eye movements show that people can miss the gorilla even when they look directly at it. In driving, we believe that if we are looking at the road and keeping our hands on the wheel, we’ll notice if anything important happens. The reality is that we hardly ever have complete awareness of our surroundings, and distractions like carrying on a phone conversation even further restrict what we notice.

State laws barring the use of handheld phones will not solve the problem. The limitation that makes it dangerous to phone and drive isn’t on our physical dexterity — it’s on our mental capacity. The distracting component of talking on a cellphone has nothing to do with taking our hands off the wheel.

Experiments testing the distracting effects of phones consistently find that driving with a hands-free headset is no safer than using a handheld one. Indeed, both have roughly the same distracting effect as driving while under the influence of alcohol.

The distraction actually comes from devoting attention to the conversation. Whenever we split our attention, we are less likely to notice an unexpected event, like a child running into the street, or a motorcycle entering our lane, even when these unexpected events are directly in front of us.

Legislation that is based on intuition rather than established science will not have the desired effect. In fact, laws that actively promote the use of hands-free headsets as a way to increase safety might paradoxically make people less safe. They give drivers a false sense of confidence that they can safely hold a hands-free phone conversation without diminishing their ability to notice the unexpected gorillas on the road.

Daniel Simons, left, is a cognitive psychology professor and head of the Visual Cognition Lab at the University of Illinois. Christopher Chabris is a cognitive psychology professor at Union College in New York. They are the co-authors of “The Invisible Gorilla, and Other Ways Our Intuitions Deceive Us,” published by Crown.