"And you don't get any better with practice," Strayer adds. In his lab, subjects who reported they use a cellphone a lot when driving "show every bit as much impairment" than those who do so infrequently.
Although no studies looked at the safety of cellphone chatter by drivers of manual-transmission cars, Strayer acknowledged that stick-shifters may reap immediate safety improvements by switching to a hands-free device for cellphone calls. But he cautions that, in principle, that would merely bring these motorists up to "the same level of impairment" as automatic-transmission drivers talking on cellphones.
He says cellphone bans that exempt hands-free devices "are half-measures that aren't really taking into account the available scientific evidence. And it's not just one source of evidence," he adds; in recent years, dozens of studies, using a wide range of methods, have concluded there is no difference between driving performance of people using hand-held phones and hands-free devices.
For instance, in a 2005 Australian study published in the British Medical Journal, researchers interviewed, during a 27-month period, 456 hospitalized cellphone users who had each been involved in a crash. The scientists combed the drivers' call records to see how cellphone use affected their driving. Whether they talked hands-free or with a phone clasped to their ear, the result was the same: During calls, and for 10 minutes after their completion, a driver's likelihood of crashing shot up fourfold.
In the lab, multi-tasking drivers fare little better. A recent study showed powerfully how doing two seemingly simple tasks can overload the brain and cause errors of judgment.
Marcel Just, a neuroscientist at Carnegie Mellon University in Pittsburgh, conducted brain imaging of 29 young adults to gauge the cognitive demands of simultaneously driving and listening. Lying in a functional magnetic resonance imaging machine, the subjects steered a simulated car down a winding road. On a second run, they steered the car while listening to general-knowledge statements and identifying them as true or false.
The study, published in April in the journal Brain Research, found that subjects who were allowed to navigate undisturbed showed robust activity in the brain's parietal lobe, a region long associated with spatial sense, distance calculations and judgments that require a person to calculate his whereabouts in a broader physical environment. When the task of listening to the sentences was added, blood flowed to different parts of the brain generally involved in the processing of language. As those language areas came alive, activity in the parietal lobe declined by almost 40%.
