Consider a parking lot with a single entry and exit gate. Two pairs of sensors$,$ SF and SB$,$ are used at the gate. When a car is between the photo transmitter and the photo receiver, the light is blocked, and the corresponding sensor output is asserted to $1.$ For entrance, when both sensors are unblocked, SB SF$="00".$ As the car enters, Sensor SB is blocked, making SB SF$="10".$ When both sensors are blocked, SB SF$="11".$ As the car moves forward, first Sensor SB is unblocked, resulting in SB SF$="01".$ Then, sensor SF is unblocked, making SB SF$="00".$ For exit, the sensors are blocked in the reverse order.