If the claxon goes off, the driver has a few seconds to acknowledge it by pressing a button. If he or she does not acknowledge, the brakes are applied automatically. And if the driver acknowledges the warning horn, a black circle on the display lights up with yellow segments. The sunflower is a visual reminder to drivers that they have acknowledged going through a signal other than green, and take must take responsbility for taking appropriate action. The driver in the Southall rail crash, where seven people died, would normally have had two warnings that he was approaching a red signal, but in that case the AWS was not working.
Computerised equipment is installed in both the train and at key points along the track.
The system is designed to stop trains travelling at up to 200mph from going through danger signals. In addition, monitoring devices between tracks register the train's speed and activate its brakes if it is going too fast. It is already operational on the Chiltern and Great Western lines, and its main functions were scheduled to be installed on the West Coast Main Line. Some experts have said that ATP would have prevented two fatal rail crashes - one at Purley in 1989, and the other at Cowden in Kent in 1994. The Hidden inquiry into the 1988 Clapham Rail crash, which claimed 35 lives, recommended that ATP should be introduced across the country's entire rail network.