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 the driver collapses onto the button or keeps 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 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. AWS for speed warnings The system works in the same way as for signals, except that a fixed magnet positioned between the running rails is located at the service braking distance before the speed reduction. The single magnet will always cause a horn (caution) to sound in the cab, which the driver must cancel to prevent the emergency brake applying. Beyond the magnet, a lineside warning board will advise the driver of