Giving Alzheimer's patients a battery of cognitive tests may help predict whether it's safe for them (and us) to get behind the wheel, according to a new study.

"We found that tests that involved visual perception and visual memory were particularly important in preventing driving errors," says Jeffrey Dawson, a biostatistician at the University of Iowa College of Public Health in Iowa City and lead author of the study published in Neurology.

Dawson hopes the findings will pave the way for the creation of a test that physicians could give to people diagnosed with Alzheimer's to determine if it’s safe for them to be on the road.

Alzheimer’s disease, the most common form of dementia, affects an estimated five million people in the U.S., according to the Alzheimer's Association, a nonprofit based in Chicago, Illinois. The disease appears to be caused by protein plaques and tangles that accumulate in the brain, damaging cells and chipping away at the victim's cognition and motor skills.

Dawson compared the driving ability of 40 men and women ages 51 to 89 with early Alzheimer's to 115 drivers ages 42 to 89 with no signs of dementia. (The average age of the Alzheimer's group, 75, was six years older than that of the other group, but the discrepancy was accounted for in the final statistical analysis, Dawson notes).  The researchers gave all drivers a series of tests designed to measure cognitive, visual, and motor skills. He says the Alzheimer's patients did worse in virtually all of them.

About a month later, each study participant was given a 35-mile road test with a researcher in the passenger seat giving instructions and monitoring performance. The car was also equipped with hidden cameras and specialized sensors to detect changes in acceleration, steering wheel position, and other factors used to assess their performances. The Alzheimer's drivers made an average of 42 errors, including straddling lanes and failing to go immediately after a light turned green, compared with an average of 33 errors in the healthy group (a 20 percent difference).

According to Dawson, the results of the combo of cognitive tests accurately predicted how many errors an Alzheimer's driver would commit: 50 points lower on a 400-point overall cognitive score translated to four more safety errors on the road, he says. He notes that the most telling individual exams were the ones requiring patients to recall and draw shapes and figures that researchers had shown them minutes or seconds earlier.

The ultimate goal, Dawson says, is to combine the most effective tests into one simple exam that doctors could use during routine office check-ups.

By the way, for all of you naysayers out there trying to get granny off the road, take note: People 65 and over account for 15 percent of all licensed drivers in the U.S. but only make up about 8 percent of those injured in accidents (meaning they are involved in far fewer crashes than would be expected), according to the National Highway Traffic Safety Administration (NHTSA). In addition, a new study by the Insurance Institute for Highway Safety found that crash deaths among drivers age 70 and older dipped 21 percent over the past decade.

But that's not to say they shouldn't be tested. The feds currently don't mandate special tests to gauge driving fitness among older people, according to NHTSA spokesperson Patricia Oladeinde. But some states impose their own stringent license renewal requirements. The District of Columbia, for instance, requires people 70 and over to present a doctor's note certifying they are physically and mentally up to the task, and in some cases, requires vision and reaction tests.