I tend to (ab)use the parking tickets tagging for that too, but it might be worth having either another tag or more likely subtags for:
- machines that give you a ticket for “free” that you later need to pay off before you can leave
- the machines that take your timestamped ticket and let you pay the appropriate amount of money so that you can leave.
- machines that check you’ve paid before they open a barrier.
- versions of these that use just you car’s registration and ANPR instead of physical tickets (probably just an extra tag).