Americans drove 9.6 billion fewer miles in May 08 than May 07. How do they find out how many miles we drive?

Question

How is that stat calculated? or is it just a figurative estimate?

http://money.cnn.com/2008/07/28/news/economy/driving/index.htm

Answer 1

toll booths, gas usage (taxes paid for your gas) - just a couple of tell-alls for you.

Answer 2

It's just a ' Guestimate '...they figure an average mile per gallon and look at how many gallons of gas were sold during that month

Answer 3

It could be data collected from toll booths, traffic monitors, gas stations, etc.  Also, people who deduct mileage on their tax forms put their beginning and ending odometer readings; although I don't know if this information is utilized.