Talking & Driving

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Hands-free cell phones may be dangerously distracting to drivers.


The risks of hands-free phones. I'm Bob Hirshon and this is Science Update.

Several studies have found that hands-free cell phones affect drivers just as much as hand-held models. A recent experiment may explain why. Harvard Medical School psychologist Todd Horowitz and his colleagues found that normal hands-free phone conversations impaired visual attention. But listening to a novel over the phone, or simply repeating the caller's words, had no effect.


It's not listening. It's not moving your mouth and making sounds and making words. It's that intervening bit, where you have to actually think about what you're going to say and generate some new content for whoever you’re talking to.

Of course, this also happens in conversations with passengers. But Horowitz notes that those people can at least see what's happening on the road, and tend to clam up when the driver needs to concentrate. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

Several states and municipalities have banned drivers from using hand-held cell phones. Statewide bans are in place in California, Connecticut, New Jersey, New York, Washington, and the District of Columbia. Yet all of these states allow most drivers to use hands-free cell phones or Bluetooth devices.

By allowing hands-free phones, the laws imply that cell phones distract drivers mainly by tying up their hands. California's Department of Motor Vehicles website, for example, notes that all car phones must have a hands-free earpiece, and that drivers are “strongly urged not to dial while driving.” However, a number of scientific studies have found that hands-free cell phones impair driving skills just as much as hand-held models. If that's the case, then there must be something else about using the phone that interferes with driving. But scientists haven't been able to agree on what exactly that is.

In Horowitz's study, volunteers participated in a test of visual attention, which involved tracking moving objects on a computer screen. When participants had to carry on an everyday conversation over a hands-free phone, they performed worse than they did with no distractions. That suggested that it wasn't just holding the phone that was the problem: something about either talking or listening also could interfere with driving.

Was it listening? To find out, the researchers tested how well people performed on the visual attention task while listening to a recorded book over the phone speaker. They performed just as well as those with no distractions—so listening wasn't the problem. Then, they asked the volunteers to repeat words that a caller said over the phone. Again, this didn't affect their performance on the test. So talking, in itself, wasn't the problem either.

Finally, they had the volunteers try the computer game again, but this time, they had to answer riddles and brainteasers over the phone at the same time. In this trial, the volunteers' scores plummeted. In fact, they performed even worse than they did when they were having an ordinary conversation. That's why Horowitz's team concluded that thinking about what you're going to say was the source of the distraction: only people who had to think about their next sentence scored worse than people who had no distractions, and within that group, those who had to think harder scored even lower.

In theory, then, talking to the passenger in the seat next to you should be just as distracting as talking on a hands-free phone. You may find this hard to believe, since drivers have been talking with passengers since cars were invented. It's possible that all other things being equal, drivers are more likely to have accidents when they're talking with passengers than when they're not. This would be nearly impossible to measure in real life, although further lab studies could indicate whether it's at least a potential problem.

However, Horowitz also points out a key difference between talking to a “live” passenger and talking to someone on a phone: passengers who are in the car with you can tell when you need to concentrate—for example, when you're merging onto a freeway in heavy traffic—and when it's safer to chat. They may instinctively shut up when the road gets difficult, or at least respond more quickly to social cues from the driver. Horowitz calls this difference “situational awareness,” and it might explain why car conversations never posed a major problem until they involved people who weren't actually in the car.

Now try and answer these questions:

  1. Why do you think so many hands-free phone laws assume that holding the phone and dialing were the main distractions to drivers?
  2. What does Horowitz's research suggest is the key distractor to drivers on hands-free phones? How did the scientists reach this conclusion?
  3. Why would it be so hard to determine if talking with passengers resulted in more real-life traffic accidents? (If you're stuck, try designing a study that would answer this question, and then notice its flaws.)

You may want to check out the January 2, 2009 Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: a listener asks, "How can it be 'global warming' when it's so cold outside?" And the chilling effects of deadly disease on the climate of the past.

Going Further

For Educators

The 2003 research article Does Cell Phone Conversation Impair Driving Performance? presented by the National Safety Council, describes various findings regarding the safety of cell phone use while driving.

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