OK, here's one I haven't seen before. On our way to a meeting in Irvine, some coworkers and I stopped at the Spectrum for lunch. Upon entering the parking garage, we saw this:
(please excuse the image burn, the bright-ass display was too much for my wee camera phone).
Here's the thing I found fascinating: The numbers would jump up and down pretty quickly, in a clearly non-human manner.
In other words, this system is automated, with the number of spaces determined by computer. Others in the car were guessing sensors in the garage, but I'm betting its based on the feed from the various security camera domes I saw. The simplest thing would just be to have the computer try to "count" cars entering and leaving the various levels, in order to keep a running total of the cars. I imagine that trying to visually inspect each space (via machine vision) to determine if a car is actually parked there is unnesessarily complicated.
There was too much math for me to get very far in the computer/machine vision classes I tried to take in school, but from what I know about it, this seems like an interesting and straightforward, if not altogether easy, piece of tech.
A brief search for info from Daktronics (the name on the sign), reveals little about the source of the data, only the nature of the display system.