My dad did a driving assessment two years ago, when he was 80. He was told that he drove with confidence and flair, but in the relatively short time since then we have really noticed his driving ability going downhill.
I wouldn't say that he's dangerous yet, but at this rate it's only a matter of time before he reaches that stage and will have to stop driving.
As others have said, the car is their lifeline, and being without it doesn't bear thinking about for my parents. They desperately want to avoid moving house, but the nearest shops / services are over a mile away, and both my brother and I live too far away to provide any practical help on a daily basis. It worries me a lot