something like that in the BWI Airport garage. There are sensors mounted above each spot with a red and green LED on the bottom. If there is a substantial metallic presence detected below the sensor, the red LED shows; if no large metal object, the green one is lit. The main alleyways in the middle of the garage have little boxes on the ceiling by each side aisle showing a green LED if there is at least one empty space in that aisle, red if all are full. You can look down the main aisle and see immediately if there are any available spots in that section, drive down to a green LED and look down the side aisle and find the green LED on the ceiling over the empty spot. If all are taken on a particular level, a sign on the entrance ramp will tell you to go up or down to the next level. Sure beats driving up and down every aisle for hours looking for non-existent parking spots. Of course, there are still fist fights when two people see the green light and come careening in from opposite directions...
dw

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