It's a difficult decision, we have a similar issue with 87 year old fil. He stays in a very small village with a minimal bus service so taking away his car would leave him housebound. He's finding it very hard after Mil died in October, so we haven't suggested anything about the car. He pops down to next village (5 miles) every morning for his papers and lunchtime roll, other than that he goes for his shopping once a week, the doctor or visits the cemetery. He's not bad behind the wheel, just s bit more hesitant perhaps at junctions. I would leave your dad to his own devices as long as he's not s danger to himself or other road users. I think he'll know himself when to stop.