when should Alzheimer's patients stop driving?

Researchers are working on some sort of test to definitively say when a given AD patient should stop driving.  "Typically, specialists say, patients gradually scale back their driving, avoiding busy freeways or night trips or left-turn intersections."  My dad did that.  He stopped leaving town and driving on the highway. Then he wouldn't drive on Route 5 (not a highway, but a busy major street in our town).  He would actually drive up the hill from his house, across town, then down the same hill on a different street just so he could cross Route 5 quickly to get to my house rather than drive a few blocks on it! 

Working on ways to help similar patients, Dawson's team (of the University of Iowa.)... developed an intricate behind-the-wheel exam: A 35-mile drive through rural, residential and urban streets in a tricked-out Ford Taurus able to record just about every action the driver takes, much like an airplane "black box" does. Lipstick-size video cameras were positioned to show oncoming traffic, too.

I've seen things like that--Mythbusters had a similar rig when they tested how distracting talking on the phone is while driving.

The results, reported in the journal Neurology (PMID: 19204261), are striking. On average, the Alzheimer's drivers committed 42 safety mistakes, compared with 33 for the other drivers. Lane violations, such as swerving or hugging the center line as another car approaches, were the biggest problem for the Alzheimer's drivers. They performed 50 percent worse.

I see elderly people all the time driving like this. They get on the highway going about 20 miles an hour and hug the line. Where are these people's families?
As part of the study, they also did various memory and neurological testing on the people to see if any test could predict the outcome of the driving test.  

Standard neurologic tests of multitasking abilities did (make a difference), ones that assess if people's cognitive, visual and motor skills work together in a way to make quick decisions. ...Alzheimer's patients ... who scored worse than average tended to commit about 50 percent more errors on the road, Dawson says.

But they weren't able to come up with a simple test any doctor could administer, yet.
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