And just to add a practical example, the Massachusetts Turnpike has been fully paid for, by tolls, for years now, but the toll booths have not gone away, and that money is not being used to provide maintenance.
You hear this sort of thing a lot. It's actually a myth. The cost of keeping a highway like the Massachusetts in a state of good repair are enormous.
Think of it like a new car. It's never really "fully paid for" even if you don't owe a penny to a bank or auto finance company, is it?
Ditto Ill-Annoy’s tollways.