Use an ATmega microcontroller to drive a traffic signal based on garage door position and parking sensors.
Garage door position sensors are controlled by Hall effect sensors read by microcontroller, one at top of door travel (DoorState.OPEN), one at end of door travel (DoorState.CLOSED). When neither sensor is in range of door, it is considered to be (DoorState.MOVING).
Parking sensors are ultrasonic proximity sensors whose value is read by microcontroller. Distance thresholds are adjustable. There are three ParkingStates -- NOCAR (no car is present), NEAR (car is detected, but not "all the way up". This state triggers the yellow lamp), and STOP (very close -- the car should stop. This state triggers the red lamp).
Several "Modes" available. Change traffic signal based on garage door alone, a combination of garage door and parking sensors, or just cycle between colors.
Check out the repo at GitHub