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Approaching emergency vehicles (primarily Fire Department vehicles and ambulances) can override the normal pattern of traffic signals in some circumstances. Both the emergency vehicle and the traffic signal must be equipped with the appropriate devices, and only some cities and certain intersections have such devices installed. One of the most common is the "Opticom" system, basically recognized as a very fast-flashing white strobe light mounted at or near the top of the emergency vehicle (not the "wig-wag" flashing high-beam headlights). A small receiving unit mounted on the traffic signal pole receives the "strobe code" and turns traffic lights green for the approaching emergency vehicle and red in all other directions.