I have an aunt who drives down a well traveled road by the community college going about 5 miles an hour hitting the brakes for no reason. Well, unless you consider that her macular degeneration, osteoporosis might have something to do with it and that she's well into her 80's. She creates a huge backup everytime she goes out and is annoyed when they beep the horn ("What's their problem"). Has been in several accidents, all of which resulted in the other driver getting a ticket. However, we know that her driving has created an unsafe environment and she should be the one charged but it's never the case.
I've found thru AAA a SeniorDrivers.org website there are several tools like "Roadwise review", "Drivers 55+" and How to help an older driver" that were helpful in addressing the subject of diminished ability and driving capabilities.