But it is not impossible, for a very select few: 2.5% of subjects (whom researchers dubbed "supertaskers") were able to successfully operate a driving simulator and chat on a cell phone simultaneously without noticeable impairment.
The study's authors say the findings may lead to further understanding of the brain's capacity, as it challenges current theories about multitasking and shows there are fewer of them than we might think.
"Given the number of individuals who routinely talk on the phone while driving, one would have hoped that there would be a greater percentage of supertaskers," said Dr. Jason Watson, who with Dr. David Strayer authored the study.
"And while we'd probably all like to think we are the exception to the rule, the odds are overwhelmingly against it."
He indicated that the chances of someone being a "supertasker" were as good as flipping a coin and getting five heads in a row.
Previous studies have measured impairment of drivers while engaged in cell phone conversations. The National Safety Council estimates that 28% of all accidents and fatalities on U.S. highways were caused by drivers using cell phones.
Six states and the District of Columbia currently ban the use of handheld cell phones while driving, and an additional 14 states also ban texting while driving. In January Transportation Secretary Ray LaHood said.
In this latest study, the University of Utah psychologists analyzed the responses of 200 participants, who were first tested in a driving simulator, and then again driving while also engaged in a hands-free cell phone conversation involving word memorization and math problem-solving.
The researchers measured reactions in braking, the drivers' following distance on the "freeway," memory, and math computation.
Results showed that for 97.5 percent of the subjects, performance suffered across the board when both driving and talking on a hands-free cell phone.
Reaction to hit the brakes was slowed 20 percent. Drivers also failed to keep pace with traffic, as following distances increased 30 percent. Their performances in memorization declined (by 11 percent), and math performance dropped as well (by 3 percent).
Yet for the "supertaskers," there was no change in their braking times, following distances or mathematical ability - and their memory improved by 3 percent.
Researchers found this small group of "supertaskers" also displayed better performance when performing a single task than did the rest of the subjects.
The study is set for publication later this year in the journal Psychonomic Bulletin and Review.
"Psychologists may need to rethink what they know about multitasking in light of this new evidence," said Dr. Strayer, who authored a 2006 study which found motorists who talk on handheld or hands-free cellular phones were as impaired as drunk drivers.
"We may learn from these very rare individuals that the multitasking regions of the brain are different, and that there may be a genetic basis for this difference. That is very exciting."
And, Dr. Watson adds, as people strive to keep with expanding technology, "it will be very useful to better understand the brain's processing capabilities, and perhaps to isolate potential markers that predict extraordinary ability, especially for high-performance professions."
The two are currently studying fighter pilots.
For more info:
Governors Highway Safety Association: Cell Phone Laws