Where I live (Grenoble, France) there are about 10 parking garages in town. All of them have these digital signs out front telling how many spaces are free. There's even some signs on the edge of town with this same info so that someone might be tempted to go towards a garage with more free spaces
I say that's great, but it's nearly useless information! I don't want to know how many spaces are in a garage, all I need is ONE. Even if you go to one of these garages which say they are full, you can always get in by waiting a few minutes in the queue outside.
What I want to know is HOW LONG WILL I HAVE TO WAIT. I believe that it's quite easy to make an educated guess and it's that info that should be displayed instead of "full".
1) Sensors can be mounted over the queue area to determine how long the waiting line of cars is.
2) The machines which people pay at know who's paid and you can make a good estimate that say within 5 minutes, that person is going to be out of the garage.
3) Some of these garages I have seen have sensors over each car space and red/green lights and led arrows directing you to available spaces. Yes, it's quite cool. They know when a space frees up and this info too can be used to calculate when that car is going to be out of the garage.
I actually sat next to someone on an airplane last year who worked for one of the companies who designed these garage parking systems. The company was German. I told her this idea. She confirmed to me that the counters only work on the entry/exit gates incrementing/decrementing the count as people come and go. She thought it was quite interesting, I wonder if they'll do anything with it.
The added cost to one of the already equiped garages would be pretty minimal, mainly an integration job to tie the ticket machines, entry/exit gates, and overhead sensors together.