Nowadays it's fairly easy to exclude people from roads: just put up a sign that says you can only use them if you paid. (You can also use a camera and some machine learning to catch offenders; or otherwise cheap overseans workers who manually review footage.)

> Most [roads] spend most of their time with excess capacity and are not rivalrous.

Most cars sit around idle most of the time. I'm not sure what your argument shows?

> And they're usually a natural monopoly, too. Not to mention that the acquisition of land to make a road is often problematic.

That's a different discussion. Though I'm more optimistic.