Driver's AWS equipment in a Class 43 driving cab
The Advanced Warning System (AWS) is a safety feature on railroads in the United Kingdom. This safety feature was first introduced in the early 1920s, but wasn't implemented systemwide on British Railways until the late 1950s as a result of several major accidents. Trains are equipped with a sensor that indicates the current aspect of signals. Before a train reaches a signal, it runs through a magnetic system under the tracks to indicate the current signal aspect. For example, for a green signal aspect, a bell or chime is heard in the cab. Any other signal aspect causes a horn or alarm to go off in the cab. If the train crew doesn't acknowledge the horn or alarm, the train automatically goes into a full emergency brake application and locks all controls of the train unless the system is reset. When the system is reset, it displays a yellow and black display in the cab, which is known as the sunflower dial because of its shape.
This system is very similar to Positive Train Control (PTC) in the United States, but PTC wasn't even created in the United States until it was signed into law in October 2008.