I don't think an average number is very helpful here.
You have some retired people who take their car to the shops once a week. Those people, even if they were still burning leaded petrol, it wouldn't matter in the overall scheme of things.
Go to a petrol station. Ask the people at the pump how many miles they drive. You will get a much higher number, because people who drive more miles visit the petrol station more frequently. That is the number you need to use in your planning.