(Though there isn't an actual 'Road Tax' as soon many believe there is.)
So what do tax disks show you've paid? My life has been a lie.
Roads are maintained either through a small tax on gas and large vehicle (55,000+ lbs) registration (for the national level) or through vehicle registration fees at the state level mostly (though this varies a little bit from state to state).
It is also funded by road/bridge tolls, at least around the New York/New Jersey area. They've raised rates a few times over the past few years on some of em.
I misread that as Trolls, which seemed like a nice and historical way to gather taxes.
I so want to use this in a fantasy setting now -- trolls employed by the local king to gather customs duties from travellers. Adventurers could be hired by a merchant company to kill the trolls at a certain bridge before they send a particularly high-value caravan through, only to find out later that their "good deed" constituted murder of the King's officials and now they're wanted criminals.
This is actually sort of brilliant, because trolls are:
1. Somewhat unimaginative and single-minded, at least in DnD, because they're low Intelligence and low Wisdom. They just want da bux for da baws, brah.
2. They're tough, nigh immune to death from non-fire/acid sources, and if you cut them apart, you just have more trolls to deal with.
3. They'd probably be a cheap hire. "I will give you one animal of your choice for every month you watch my bridge."
4. They're intimidating. No one wants to tell the ten foot tall, insta-regenerating, man-eating monster 'no'.