page_title: Automatic Warning System
section_title: AWS at signals
hierarchical_section_title: Automatic Warning System / Principles of operation / AWS at signals
caption_attribution_description: English: A temporary speed reduction sign. Created by myself in Macromedia Fireworks MX
context_page_description: The Automatic Warning System was introduced in the 1950s in the United Kingdom to provide a train driver with an audible warning and visual reminder that they were approaching a distant signal at caution.
  Its operation was later extended to give warnings for;
  A colour light signal displaying a double yellow, single yellow or red aspect
  A reduction in permissible speed
  A temporary or emergency speed restriction
  An automatic barrier crossing locally monitored, an automatic open crossing locally monitored, or an open crossing.
  AWS was based on a 1930 system developed by Alfred Ernest Hudd and marketed as the "Strowger-Hudd" system. An earlier contact system, installed on the Great Western Railway since 1906 and known as automatic train control, was gradually supplanted by AWS within the Western Region of British Railways.
context_section_description: As the train passes over an AWS magnet, the 'sunflower' indicator in the driver's cab will change to all black. If the signal being approached is displaying a 'clear' for a semaphore or green for a multiple aspect colour light signal, the AWS will sound a bell (modern locomotives and multiple units use an electronic sounder that gives a distinctive 'ping') and leave the visual indicator black. This lets the driver know that the next signal is showing 'clear' and that the AWS system is working.
  If the signal being approached is displaying a restrictive aspect (red, yellow or double yellow in colour-light installations or a distant semaphore at caution (horizontal)), the AWS will sound a continuous horn or buzzer. The driver then has approximately 2 seconds to cancel the warning by pressing and releasing the AWS/TPWS acknowledgement button (if a driver should collapse onto the button or keep it held down, the AWS will not be cancelled). On cancelling the warning, the horn then stops and the visual indicator changes to a pattern of black and yellow spokes, which persists until the next AWS magnet and reminds the driver of the restrictive aspect.
  As a fail-safe mechanism, if the driver fails to cancel the warning in time, the emergency brake will apply and bring the train to a stand. When this occurs the red Brake demand light will flash on the AWS/TPWS Driver machine interface. The driver must now push the AWS/TPWS acknowledgement button, and the brakes will release after a safety time out period has elapsed.
  AWS is provided at most main aspect signals on running lines, though there are some exceptions:
  At through stations where the permitted speed is 30 mph (48 km/h) or less and the layout is complex. Where this occurs, these are called AWS gap areas.
  AWS magnets are not provided at semaphore stop signals (which can only display clear or stop).
  Where a line is not fitted with AWS magnets, it is shown in the Sectional Appendix.
caption: Temporary speed restriction warning board
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