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.
In six states, including California, doctors are now required to turn in people they believe can no longer drive safely. Forty-four states allow doctors or family members to anonymously report someone to motor vehicle officials, who can then order that person to submit to a license re-examination.
After learning that her father had resumed driving again after she'd expressed previous fears, McArthur called Florida's Department of Motor Vehicles to turn him in.
"I called them in a kind of panic," she said.
When Hill's doctor refused to certify that Hill could drive, he was forced to surrender his license.
"I tried, but I lost," he said.
"I think he made the right choice," said McArthur. "I think deep down he knows he made the right choice, that it was time."
"I didn't make it," countered Hill. "I didn't have a choice."
Despite increasing physical frailty and neurological impairment, Hill insisted he would get retested and drive again someday. Either that, or move to Nigeria.
To avoid similar arguments over driving, experts advise family members to raise the topic sensitively and to have the conversation over a long period of time. They also suggest having alternate transportation in place so older loved ones are not isolated at home, unable to get around.
ABC News has compiled a list of Easy-to-Use Car Services for Elderly Drivers to help you find alternate transportation in your state.