Parking Space Detector

In the center of any parking space in a city, embed a small electronic eye that detects whether no object is over it, that is, if the parking space is free. A central software application somehow (existing cell phone technology would probably work best) contacts these electronic eyes when needed, and retrieves each eye's status and location.
Car owners can now use GPS on their cell phones (or in their car) to communicate with the central software application to find a free parking space near their current location. Finding a parking space costs the user the price of an SMS.