In general, green indicates that pathing arrangements allow for the train to depart.
Yellow indicates caution, and warns the driver to be prepared to stop at the next signal.
Red indicates danger/a passing train and instructs the driver to stop.

Prior to the start of testing of the HST-P (High Speed Train - Prototype), signals were designed for use alongside services that did not require significant lead time when departing.

During 1974, the first HST power cars, Nos 41001 and 41002 (later 43000 and 43001, becoming RDB975812 and RDB975813 respectively) entered testing along the Great Western Mainline between London Paddington and Exeter Saint David's, which led to large-scale developments along the route in preparation for the delivery of production power cars in 1977.

Included in these developments was a new signalling system designed to be understood by drivers travelling
at speeds up to 125mph, the maximum envisaged speed for the InterCity 125.

This included the addition of flashing yellow to the colour range, which would indicate the proximity of a train to drivers of the InterCity 125 sets. Signals were positioned 1 mile apart, forming a system of 'blocks', which would serve as a standard unit for drivers to approximate the location of the train in front.

Single Flashing yellow indicates that the train ahead is one block away from the signal, whilst double flashing yellow indicates that the train ahead is two blocks ahead. There is no indication for three blocks ahead, as most signals only contain two apertures.

The High speed signalling system only added on to the previous system of signalling, so this information should be regarded in conjunction with basic principles.