Bad analogy, try again. No one is saying trucks don't cause more damage than cars. Just as no one is saying helmets don't help in crashes.
The politicians disagree with you, that's why I pay 10 times more for my truck than I do for my car. The engineers would also disagree and they have. Multiple studies have been done, a simple google finds many examples. The numbers range from study to study, as I would expect, but they are all reasonably close. Remember, this is an attempt at reaching a nominal number, not a specific number. You can reasonably expect South Florida to be warmer than PA by 35F. Over a long period that proves to be true. When it's 90F where I live I seriously doubt Miami is 125F. On the other hand when it's -10F here I've often seen Miami at 70F. Over a long period though delta 35F seems to about right. If we were to tax Miami for their nicer weather 35F would be the penalty...stupid and simple example but then it was a spur of the moment attempt. Interstates are different from state highways and new roads are built different from old roads but all of that can be evaluated and considered. On a state by state basis rather than national the assessment becomes even easier as that eliminates most weather and building code variables.
So basically you agree with me that the 9000 figure is bullshit and there cannot be a set number. Cool.