Door Doctor Stop & Go lights increase safety by providing visual communication anywhere a positive go / no-go signal is required. Door Doctor Stop & Go Signalling System Alerts Truck Drivers and Loading Dock Personnel to the Safety Status of Docked and Docking Trucks. The SG30 System utilizes two units – one outside of the loading dock and one located on the inside. The system is controlled by a toggle switch mounted on the inside light. (Remote switching is possible). When the outside light is flashing red the inside light is green. This tells drivers not to move the truck and indicates that loading and unloading may proceed. When the toggle is flipped, the red light inside warns dock workers to stay out of the trailer, and green tells the driver that he is free to move the vehicle. The SG10 is available for applications where opposing signals are not required. Ideal for installations utilizing external controllers such as drive thru indicators and for OEM applications.