Roads are non-rivalrous (to an extent). Meaning during off-peak times a private toll road would likely be under utilized if it were not regulated/subsidized. Another reason is that if a road happens to be the shortest path between a populous area and an in-demand destination it would likely be something close to a natural monopoly. This could potentially lead to high prices, too many billboards, etc.
https://en.wikipedia.org/wiki/Rivalry_(economics)
https://en.wikipedia.org/wiki/Natural_monopoly