diamond. This is intended to avoid confusion between transit signals and main traffic signals at intersections where both sets are visible. In Russia, transit signals consist of 4 white lights, forming a "T". The bottom light is always illuminated when transit vehicles have permission to proceed. The top 3 lights represent the direction in which transit vehicles are permitted to proceed. For example, the left light allows transit vehicles to turn left, the middle light allows transit vehicles to proceed straight, and the right light allows transit vehicles to turn right. Multiple lights may be displayed at the same time, to allow transit vehicles to proceed in multiple directions at the same time. Transit signals are mostly used to give trams priority, but sometimes are used for buses and trolleybuses too. Emergency priority signal At many intersections in the US, intersections use traffic signal preemption to give priority to emergency vehicles. These preemption applications often include an illuminated "notifier" signal. A notifier is a secondary lighting device usually mounted independently of the traffic signal, such as a standard or strobing light bulb in an omni-directional enclosure or spotlights aimed at oncoming traffic lanes. The colors of these secondary lighting devices vary regionally depending upon the operational policies of the local traffic management and emergency service agencies. Reverse-side red indicator Some jurisdictions use special small blue lights on the reverse of signal heads to indicate a red light lit on that head. They are used to communicate the presence of a red signal to police so they can view the situation without having to traverse the intersection. Other jurisdictions simply drill a small hole in the red signal visor to allow police to see the status of the signal from a wide angle (but not directly opposite the signal). In France and, perhaps, Vietnam (which was previously a French colony by the name of Indochina), the back of the red lights are sometimes designed with a red cross to indicate that the light is red, so that opposing left-turning drivers know that oncoming traffic has been stopped. Pre-warning of a changing signal Between green and amber In Austria, Cambodia, China, Estonia, Latvia, Lithuania, Montenegro, Russia, most of Israel, Malaysia, parts of Mexico, some places in Thailand, Turkey, parts of Serbia, and in certain other parts of Europe, the green lights will start flashing at the end of the Go or Turn