Feb. 2, 2011, 2010 — -- Robert Hill and his daughter Cathy McArthur are in the midst of a classically fierce clash. Like many Americans with aging parents, McArthur doesn't think it's safe for her 84-year-old father, who has Parkinson's disease, to drive anymore.
Watch 'World News With Diane Sawyer' for more on this story tonight on ABC at 6:30 p.m. ET
"We have a disagreement. If it doesn't go away, and I'm not able to drive, I will leave the country," said Hill. "I'll go to a country where they don't have driver's licenses. If I have to go to Nigeria, or some place like that, I'll go."
Determined to keep his independence, the retired Air Force pilot continues to drive -- behind his daughter's back -- even though he shakes and experiences occasional blackouts.
"I don't find a compelling argument from anybody that I shouldn't drive except my daughter," said Hill.
"I was in the car with him one time last year. I noticed that he did have some difficulties staying in his lane," McArthur said. "He didn't use his blinkers. He crossed over some traffic, was speeding up too fast toward the light -- a lot of little things that were kind of telltale signs."
McArthur worries not only about her father's safety but also about the other people on the road. And her fears are not unfounded.
At the MIT Age Lab in Cambridge, Mass., researchers study how age and infirmity can affect driving skills.
"You have neurological changes in the brain, which is just processing time, sending signals from one nerve to another," said Joseph Coughlin, director of the lab. "Some get slower more than others."
Making a left turn can be especially troublesome. With every year after age 65, the odds of getting into a car crash while attempting to go left increases by 8 percent. With limited mobility to look in both directions, it becomes more difficult to gauge speed, distance and timing, especially when there's oncoming traffic.