We lucked out a little bit on this. At 84, my mother in law already had a dementia diagnosis, but she lived in a rural area and she was very isolated. It was half a mile from her condominium to the nearest public road. Her entire life revolved around driving, and there were no relatives within 80 miles. By the time she stopped driving, it was already four years since the last time any of her children or grandchildren had gotten into a car with her behind the wheel, but she was still driving hundreds of miles to visit them. She had already decided to move to live near us when she started to get lost periodically – driving aimlessly for up to an hour, then relying on help from strangers to get her home. When she moved, she fully intended to keep driving. But we went into gruesome (and only somewhat) exaggerated detail with her about what it would cost to park and to insure her car in the big city. (It would have been her largest expense after rent.) She reluctantly agreed to sell her car before she moved.
My mother and my father-in-law both had medical conditions (Parkinson’s and a leg amputation) that stopped them from driving before they became dangerous. There was a bright line, none of this gray area, and they knew they had to rearrange their lives on a new basis. My father-in-law could have learned to drive with a prosthesis or with special controls, but he never did.
My father was the tough one. He had been getting into small accidents regularly for years. His firm moved, and didn’t give him a parking space in the new office. My sister, who lived a few blocks from him, hadn’t gotten him to give up his keys definitively, but had gotten him to cut way back on his driving. Sadly, he might still be alive if he had driven himself to the hospital after falling and hitting his head while on warfarin. He had a couple of hours of lucidity before uncontrolled brain bleeding and swelling put him in a coma, most of which he spent trying to contact my sister and waiting for her to drive him to the ER. By the time he got there, it was too late to do anything.