What a nest of vipers this problem is! My DH had a very powerful car and drove it in a terrifying way, either too fast or too slow. It was sporty and that annoyed all the young drivers so he was beset by bullies and would respond angrily. I was terrified and offered to drive my car everywhere. He never got the idea of roundabouts, or pedestrian controlled crossings.
I saw his GP. I wrote to the DVLA. The law seems to be that people have to refer themselves to the DVLA. Doctors won't, unless the person has had a recent stroke or TIA, when they have to stop driving for a month. They say it would destroy the patient's confidence in them and might stop people telling them about symptoms.
I have known of people paying thousands of pounds to repair damage to other drivers' cars, to avoid telling their insurance company.
There are courses and informal tests available in a lot of places, and perhaps saying, 'go on, prove us all wrong, do one of these courses!' might work to encourage some to take one. DH was frustrated that my DF was still driving at 92, utterly safely, while he was only 75 and the family were all complaining. It is this variability that is the problem.
In more rural areas there are no buses as an alternative way to get around and often families are at work or live too far away to be of help. Taxis are expensive, but selling a car and not paying insurance, car tax, and maintenance saves a lot of money, so taxis could be afforded.
It robs independence, and often in older households it takes away the last function the 'man-of-the-house' has. Of course they fight against losing their car. But it is a lethal weapon and we all feel the responsibility to the rest of the community to stop someone who is a danger from getting behind the wheel.
Perhaps we should start one of those Downing Street petitions as Gransnetters, to get the law changed so that anyone can report someone to the DVLA or police. Perhaps insurers could do it if someone had too many claims?
I hope someone will tell me if they ever get worried about my driving. I'd stop in an instant, in case I hurt someone.