Why has the insurance industry been so cool to mileage-based pricing?
For the same reason that car rental companies started offering unlimited mileage on their rentals. The cost of tracking the mileage and assessing variable rates would be enormous. And when all is said and done they'll find that the people who drive the fewest miles are actually the highest risks.
The best driver I've ever met is a salesman who puts about 30,000 miles per year on his car, almost all of it in the worst region in the country for drivers -- the New York metropolitan area. He hasn't had an accident in years, but this kind of scheme would cost him a fortune.
USAA Auto insurance simply asks the customer how many miles per year. This is done on a yearly basis.