Startup Idea : See available lots [Parking Lot]

On the entrance of the parking lot from a shopping or another thing, you can see in a TV/Monitor all the lots available/not available. Then, you can find your lot and go there. It will need some ultrasonic sensor in each slot and a communication interface with Arduino, maybe. So, there's a 'problem': you see an available lot, but when you go there, there's a car. Because on the moment you saw the lot, nobody was there, until this time, another car just parked there.