When someone is talking to you, your brain is listening, processing and thinking about what's being said _ even if you're in the driver's seat trying to concentrate on traffic.
That's why drivers get distracted during cellphone conversations, even when using hands-free phones, researchers say. It's also part of the reason why the National Transportation Safety Board made a recommendation this week it knows a lot of drivers won't like _ that states ban hands-free, as well as hand-held, cellphone use while driving.
It's not where your hands are, but where your mind is that counts, NTSB chairman Deborah Hersman told reporters.
The board doesn't have the power to force states to impose a ban, but its recommendations carry significant weight. And, judging from the public reaction, they've already started a national conversation on the subject. NTSB has been swamped with calls, emails and tweets from drivers both praising and condemning the action.
It's the proposed hands-free ban that has generated the most controversy.
What's next? No passengers? No kids? No tuning the radio? Maybe NTSB will ban driving altogether, was the tenor of the response on Twitter.
The scientific evidence, however, is generally with NTSB, researchers said.
"There is a large body of evidence showing that talking on a phone, whether hand-held or hands-free, impairs driving and increases your risk of having a crash," Anne McCartt, senior vice president for research at the Insurance Institute for Highway Safety, said.
Jim Hedlund, a safety consultant and former National Highway Traffic Safety Administration official, recently examined 300 cellphone studies for the Governors Highway Safety Association. He couldn't recall a single study that showed drivers talking on a headset or hands-free phone were at any less risk of an accident than drivers with one hand on the wheel and a phone in the other.
A similar analysis for the government of Sweden recently came to the same conclusion: "There is no evidence suggesting that hands-free mobile phone use is less risky than handheld use."
What's missing is hard evidence that accidents are increasing because of cellphone use. One reason is that U.S. privacy laws have made it difficult for researchers to study whether cell phones were in use in accidents in the U.S. The two large studies that have been done _ in Canada and Australia _ found drivers were four times more likely to have a crash if talking on a cellphone. It didn't matter whether the cellphone was hands-free or hand-held.
But that hasn't translated to an increase in highway fatalities in the U.S., where they hit their lowest level since 1949 last year.
Of 6,000 drivers surveyed by the highway administration, 40 percent said they don't consider it unsafe for drivers to talk on a hands-free cellphone. Less than 12 percent said that about a hand-held phone.
Marcel Just, director of Carnegie Mellon University's Center for Cognitive Brain Imaging, isn't surprised.
It's counterintuitive to think that hands-free talking is dangerous because people don't have any sense that their conversation is draining brain power away from driving, but that's exactly what's happening, he said.
Just is the co-author of a 2008 study that used driving simulators to test the performance of drivers not engaged in conversation and drivers who could hear someone talking to them through headphones. Drivers took the simulator tests inside an MRI (magnetic resonance imaging) machine that recorded images of changes in their brains while driving, including which areas of the brain were used for driving. The amount of the brain devoted to driving was 37 percent less in drivers who could hear someone talking to them than for drivers not using cellphones.
"The human mind can multitask, but each task is performed with less brain power and lower proficiency," Just said.
The driving simulators also showed a deterioration of skills on the part of drivers who could hear someone talking to them, including weaving between lanes and edging over the side of the road.
"When someone is speaking your native language, you can't will yourself to not hear and process it. It just goes in," Just said. Even if a driver tries to ignore the words, scientists "can see activation in the auditory cortex, in the language areas (of the brain). "
Accident investigators have seen cases of drivers talking on hands-free phones whose minds are so engrossed in their conversations that they ran into something plainly visible.
In a 2004, a bus driver taking students on a class trip drove his 12-foot-high bus into a 10-foot, 2-inch-high bridge arch in Alexandria, Va., peeling off the roof of the bus. There were signs warning drivers about the height of the bridge, and the bus driver was familiar with the route. He also saw a bus in front of him change lanes to avoid the low arch. But the bus driver, who was talking a hands-free phone at the time, drove right into it.
"There is a standard code for crash investigations called roughly `look, but didn't see.' In other words, I was looking in the right place, but I didn't register what was there," Hedlund said.
Of course, drivers don't have to be using cellphones to have conversations _ they talk with passengers all the time. But talking to an adult passenger doesn't involve the same risk as a phone conversation, researchers said. That's because passengers are engaged in the driving experience with the driver. If they see a danger, they'll usually warn the driver. Passengers also tend to instinctually adjust their conversation to the level of traffic and other difficulties confronting the driver.
There are lots of other things that go on in cars that are risky: eating and drinking, tuning the radio, studying maps and applying makeup, for example. Just like talking on the phone, most of those things involve a choice by the driver.
As for the screaming toddler in the backseat demanding attention, "some things are just part of life," McCartt said.
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Governors Highway Safety Association: http://www.ghsa.org/