[wbsgrepit]

There are protections to this type of abuse in just about every comercial parking system -- so much so it is not even listed in advantages/features for most of their marketing materials. It is commonly refereed to Single Entrance Gate or Stateful Egress; basically the card is tracked in a stateful database and is either in use (it has been used to enter but not yet to exit) or not in use (it has exited since its last entry). If you attempt to use the card for entry when it is in the "in use" state it will deny and log the action.

Edited to add: For the next common "game" on this line "why dont you just walk past and card swipe exit" you will note the cameras positioned at the card terminals at all of these lots for this issue. Basically they will have a record of: your license plates for the entry/exists (the sipes and video systems have timecodes that can be matched) and of your person walking by on foot to trigger the exit state.

[jacobush]

At our place there are also some kind of car detector, presumably magnetic coils in the ground. You can't walk past.