Are 'Hands-Free' Phone Calls Really Safer for Drivers?
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Talking on your phone while driving — even if you use a hands-free connection — is still dangerous, a new study from England finds. 

Conversations of any kind can be distracting if they require a driver to use mental imagery, according to the study. 

"A popular misconception is that using a mobile phone while driving is safe as long as the driver uses a hands-free phone," Graham Hole, a psychology lecturer at the University of Sussex in England and an author of the study, said in a statement. "Our research shows this is not the case." [Understanding the 10 Most Destructive Human Behaviors]

Rather, the findings of the new study suggest that when drivers are engaged in a conversation and are using visual imagery behind the wheel, their brains fail to recognize all of the hazards on the road ahead. 

The study included two experiments. In the first experiment, 60 people, divided into three groups of 20, completed a simulated driving course with a series of road hazards. A video of the course was shown through the windshield of a nonmoving car, and the researchers measured when the drivers stepped on the brake pedal. 

The first group of people completed the course with no distractions. In the second group, the people were asked true or false questions while they carried out the driving simulation. These questions required the people to use visual imagery in order to answer. For example, a true or false statement might say, "In a rowing boat, the rower sits with his back to the front of the boat," prompting the driver to picture a rowboat in his or her mind. The third group was also asked true or false questions while completing the driving simulation; however, these questions did not involve visual imagery, according to the study. One true or false statement, for example, was "The official language of Mexico is Spanish."

The researchers found that the people in the undistracted group detected the most hazards in the driving simulation, and the people who were asked questions that required mental imagery detected the fewest hazards during the experiment.

In the second experiment, 46 people, divided into two groups, completed a similar driving task while the investigators tracked the participants' eye movements. The first group was given no distractions during the experiment. The second group, however, was asked to imagine a 3 x 3 grid, and picture themselves positioned in the center square, according to the study. Then, they were told to "travel" around the grid in response to instructions (move left, right, up or down) and tell the investigator which square they ended up being in at the end.

The researchers found that the drivers in the second group had slower reaction times to road hazards, and were more likely to "look but fail to see" hazards. In other words, the eye-tracking data showed that the participants looked at the hazards but did not react to them. In addition, the drivers in the second group focused more on the portion of the road directly in front of them than the road in their peripheral vision, and they missed some of the hazards in their peripheral vision.

Taken together, the two experiments show that when drivers engage in conversations that spark their visual imagination, they are less likely to detect hazards on the road and more likely to focus on a smaller portion of the road ahead. [10 Things You Didn't Know About the Brain]

It's possible that the brain has a limited capacity for visual processing, according to the study. When a driver is tasked with both recognizing hazards on the road and mentally picturing some aspect of a conversation, that visual imagery "competes for processing resources" in the brain, Hole said.

"Conversations are more visual than we might expect, leading drivers to ignore parts of the outside world in favor of their inner 'visual world,'" which is a concern for drivers' safety, Hole said.

But drivers don't necessarily need to drive in silence.

"Chatty passengers tend to pose less of a risk than phone conversations," Hole said.

Passengers "will usually moderate the conversation when road hazards arise," Hole said. But "someone on the other end of a phone is oblivious" to what's happening on the road, and keeps talking, he said.

"The only 'safe' phone in a car is one that's switched off," Hole said.

The study was published today (June 7) in the journal Transportation Research Part F.

Originally published on Live Science.