Nobody gets a clear announcement that a person should stop driving. What you get is a car that keeps picking up marks, a route that takes longer than it used to, and a conversation where the answer is always a little different. The useful move is to stop relying on memory and write the signs down.
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What belongs in the log
Keep one line for each event: the day, the place, what happened, and who saw it or heard about it. Include the small things that are easy to explain away. A new scrape on the bumper. A curb tap in a parking lot. A side mirror folded in that wasn't folded before. A car parked crooked across two spaces. A trip that ends with him calling you from a store he has driven to for years. A turn he made that didn't look like a turn he would have made. A notice from an insurance company or a police or court letter, if one arrives.
Photograph new damage when you see it, and keep the photos in the same place as the log. The car is the best witness you have, and it doesn't argue.
How to tell it is a pattern
One dent is a parking lot. When the entries repeat in the same place, or the same kind of event keeps happening, you have a pattern, and a pattern is what the next step is for. Watch also for changes in how he talks about driving: he avoids certain roads, he suddenly drives less at night, or he says he'll stop and then doesn't. Write those down too, but keep them factual. What he said and what happened, not what you think it means.
Who to bring it to
The log is for a specific person, not for a family argument. His doctor is the right first stop for a question about whether driving is still safe for him, and the log gives the doctor something concrete to work from. Bring the entries as a list. Ask directly whether a driving assessment makes sense, and whether one is available near you, since some areas offer one through rehabilitation programs. Your state's driver licensing office can tell you what a review or re-test involves and how it is started. An insurance agent can answer questions about coverage if the car is involved in a claim.
Frame it around the car and the cost
Dad takes his keys seriously, and for a lot of men a car is the last piece of their own life they control. You will get further by talking about the damage, the repair bill, and the safety of the people on the road than by talking about his skill. Ask him to come along to the appointment. Ask him what he'd want to know. You are trying to keep him in the decision, not out of it.
Next move
Tonight, start the log with the last three things you noticed, even if they seem minor. Photograph any new damage on the car. Then call his doctor's office, ask for an appointment focused on driving, and bring the list. If you need local options for the next steps, the Local Help in St. Louis page is the place to start.