Roads also break down on their own, though. The large majority of roads will see very little or no semi traffic, and their upkeep is entirely for the sake of light vehicles. In the north, where freeze-thaw costs much more, the impact of trucks is much smaller.
Then mandate that vehicles over X weight can't use gasoline unless they have a special exemption. Not ideal, since the revenue still only vaguely goes towards the highway system and doesn't pay for the whole thing, but hugely improved.
The exemption thing is possibly an issue but I don't think it would price out new diesel light vehicles (and IMO, that possibility is just... unimportant) if you were careful about removing the tax on pumps that aren't accessible to heavy vehicles. I believe some states do similar things already, but most of them instead have some weird reimbursement process.
One potential easy fix is to zero the federal tax on gas and make up the difference in diesel tax: https://www.eia.gov/tools/faqs/faq.php?id=10&t=10
Then mandate that vehicles over X weight can't use gasoline unless they have a special exemption. Not ideal, since the revenue still only vaguely goes towards the highway system and doesn't pay for the whole thing, but hugely improved.
The exemption thing is possibly an issue but I don't think it would price out new diesel light vehicles (and IMO, that possibility is just... unimportant) if you were careful about removing the tax on pumps that aren't accessible to heavy vehicles. I believe some states do similar things already, but most of them instead have some weird reimbursement process.