In traffic engineering, there are regional and national variations in traffic light operation. This may be in the standard traffic light sequence (such as the inclusion of a red-amber phase) or by the use of special signals (such as flashing amber or public transport signals). Flashing signals Flashing red In the United States and Canada, a flashing red light is the equivalent of a stop sign. In New Zealand, Hong Kong and the United Kingdom, paired red/red traffic lights are often installed outside fire and ambulance stations on major roads, which, when activated by the station, flash alternatively (so that at any time one red light is always showing), the purpose being to cause traffic to stop for a set amount of time to allow emergency vehicles to exit their station safely. The UK also uses an amber light which precedes the flashing red lights, and these signals are also used at level crossings, airfields and lifting bridges. Flashing amber/yellow In the US, Canada, Brazil and Australia, flashing yellow does not require traffic to stop, but drivers should exercise caution since opposing traffic may enter the intersection after stopping. This may be used when there is a malfunction with the signals, or late at night when there is little traffic. A single four-way flashing light showing only one color in each direction may be used at intersections where full three-color operation is not needed, but stop or yield signs alone have not had acceptable safety performance. Yellow lights are displayed to the main road, to highlight the intersection and inform drivers of the need for caution. Red lights supplement stop signs on the side road approaches. All-way red flashing lights can supplement all-way stop control, but all-way yellow signals are prohibited by US regulations. In Canada a flashing yellow light at an intersection that would otherwise have traffic signals indicates that the traffic signals are malfunctioning. In this case the flashing yellow is seen by all vehicles approaching the intersection, and indicates that all vehicles must treat the intersection as a four way stop. All vehicles must come to a complete stop and yield right of way as per four way stop procedure. This would continue until the traffic signals have been fixed. Flashing yellow arrow In the US, a flashing yellow arrow is a signal phasing configuration for permissive left turns. An additional aspect displaying a yellow arrow is