) -- Hands-free cellphone systems distract drivers almost as much as holding a device up to your ear -- and voice-to-text email programs are more or less the worst of all, AAA research shows.

"'Hands-free' is not risk free," says University of Utah researcher David Strayer, who recently studied distracted driving for AAA. "You can get impairments even with some of the new voiced-based

systems that allow you to keep your hands on the wheel and your eyes on the road."

Strayer and his team performed a series of experiments on volunteers to measure how a range of distractions -- from listening to the radio to answering math questions -- affect driving proficiency.

Using road tests and driving simulators, researchers monitored how volunteers' brake times and adherence to speed limits and proper following distances changed as subjects engaged in a series of side activities. Investigators also recorded participants' brain waves and eye and head movements to see how well drivers stayed focused on the road.

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Based on the findings, Strayer created a system to rate how much different activities impair drivers.

Rankings range from a Category 1 (minimal distraction) to a Category 5, which the professor describes as "off the charts." Tasks that demand roughly the same amount of brainpower as regular driving rate a Category 1, while those involving 200% the concentration levels garner a Category 2, etc.

Strayer says anything higher than a Category 2 appears dangerous, as "you're pretty minimally engaged as a driver by the time you get above that."

Here's a look at how various activities rank on his scale:

  • Regular driving. Strayer gave driving with no distractions a baseline ranking of Category 1.
  • Listening to the radio. This activity also rates a Category 1, as the professor calculated it only involves 121% of regular driving's "mental workload."
  • Listening to books on tape. Strayer estimates listening to books on tape requires 175% the brainpower of regular driving, meaning it's still a Category 1.
  • Hands-free cellphone calls. Hands-free cell calls rank a Category 2 because they require an estimated 227% as much mental work as basic driving does.
  • Talking to a passenger. Holding a conversation with another person in your car is a Category 2 task because it involves some 233% as much cognitive effort as regular driving.
  • Hand-held cellphone calls. Talking on a cellphone you hold in your hand ranks a Category 2, as it demands roughly 245% the concentration that driving with no distractions does.
  • Dictating an email with speech-to-text system. This activity rates a dangerous Category 3 because it involves an estimated 306% as much brainpower as driving a car with no side activities.
  • Doing math/memory problems. To learn whether mental distractions impair drivers more than physical ones (such as holding a cellphone), researchers gave test subjects a series of math and memory problems to solve while traveling down the road. While drivers would never do such work in real life, Strayer ranked the tests -- which involve brainpower but no physical effort -- a Category 5 because they require an estimated 500% of simple driving's cognitive effort.

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Strayer says his rankings show that laws allowing hands-free cell calls while banning hand-held ones "just aren't based on good science. They're making a distinction between two activities that seem to have the same level of impairment."

So he recommends drivers "not be lulled into thinking 'hands-free' is risk free. Driving is inherently risky -- you need to not only keep your eyes on the road and your hands on the wheel, but you need to pay attention to what you're doing."