The one near D.C. is located in the terminal parking garage at BWI (Baltimore Washington International). At this point it is a limited installation. As I understand it it uses a sensor, type unknown, to determine the presence of a car in each space. If there are open spaces iluminated arrows at the end of each row point down the row. If all the spaces are filled the display changes to either an X, or a "no free spaces" message, I ca't remember which.