Re: What about the downsides?
"not taxes for taxes' sake but specifically to fund road improvements and maintenance"
We've been here before. There used to be the Road Fund. Then it was morphed into the VED, sucked into the Treasury and the roads see less and less of it. In part new roads and bridges which the Road Fund should have financed have instead been built on a toll basis, taking us back to the late C17th.
Impose a new tax to fund road improvements and you'll see (a) even less Treasury funding for roads and (b) the Treasury getting its hands on the new tax.