In my part of the United States, the state government authorized the construction of toll roads with an agreement that those roads would eventually be handed over to the state government once the projects have been paid off.

That being said, the toll roads keep expanding so that they can continue to be operated by the private company that built them.

This is an arrangement the state government is quite happy with, because they do not have to budget for the maintenance of those roads and they are able to collect taxes on the revenue collected by the toll authority. Converting those toll roads would cause a spike in expenditures and a drop in tax revenue.

Therefore, there is a perverse incentive on both sides to keep the toll roads from being converted into public ones.

I have no idea if this sort of thing was prevalent in the past, especially in the UK. But I wanted to chime in about why several toll roads that were supposed to be paid off thirty years ago are still going strong. I suspect that 500+ years ago “build toll road, hand toll road to the public” was a lot more straightforward.