[DrStalker]

There are systems that do this; I've seen a few carkparks where each spot has a red/green light on the roof in front of it so you can tell at a glance where a free spot is. Every point where you have to choose which way ("Up to the next level or check this one?") will give the number of free spots for either choice on a little LED sign.

Westfields Bondi Junction went as far as having an iPhone app where you could put in your car's license plate and it would tell you which spot it was parked in; that caused some privacy concerns and I don't know if it's still available but the technology exists, and presumably the cost is offset by the extra metrics you get by tracking every single vehicle that uses your shopping center.