[MichaelGG]

Presumably the people creating this app have thought of this, seeing as it's like one of the critical parts of completing a transaction. With a few seconds of thought you can come up with a quick way to make sure it all works.

What might happen though is that if a car is about to leave and a non-paying car pulls up to get in, the leaving car might decide to stay a bit longer until a paying car gets first into queue.

[georgemcbay]

They might have thought of it, but there is actually no solution to it because the non-paying person waiting for the spot isn't a player in the system they are constructing and thus can't be relied upon to react in a predictable manner.

"What might happen though is that if a car is about to leave and a non-paying car pulls up to get in, the leaving car might decide to stay a bit longer until a paying car gets first into queue."

Yeah good luck with that. I've seen near brawls break out when two or more people thought they had "dibs" on an opening spot based merely on proximity and some ill-defined rules. Introduce a money component into this and that just raises the stakes on the rage response.

If I arrived at a spot where it was clear someone was ready to leave and realized he was waiting longer for a "paying" replacement to arrive to take the public parking spot, unless I absolutely had to be somewhere immediately I would queue up behind the leaving guy and block out the paying party and wait them out just out of spite for the whole concept of this model.