>You can ratchet up the tax on heavy trucks because "muh road wear" but then you'll have a world where everything is delivered in small trucks and there will be problems with that

Is that really a bad thing? If shipping via big trucks (compared to small trucks) causes $200 more road wear per year, but saves $100/yr in gas (and other expenses), why shouldn't we use small trucks?

> At the end of the day it comes down to a subjective question of which problems get which priority.

You're right, there will be some subjectivity involved, but at least the general goal is trying to be as neutral as possible. It's not unlike cap & trade for dealing with climate change - letting the market decide what's the most cost effective way.