(Outer Signal). Limitations TPWS has no ability to regulate speed after a train passes a signal at danger with authority. However, on those occasions there are strict rules governing the actions of drivers, train speed, and the use of TPWS. There are many reasons why a driver might be required to pass a signal at danger with authority. The signaller will advise the driver to pass the signal at danger, proceed with caution, be prepared to stop short of any obstruction, and then obey all other signals. Immediately before moving, the driver will press the "Trainstop Override" button on the TPWS panel, so that the train can pass the signal without triggering the TPWS to apply the brakes. The driver must then proceed at a speed which enables them to stop within the distance that they can see to be clear. Even if it appears that the section is clear to the next signal, they must still exercise caution. Whilst critics claim TPWS is a cheap solution, and puts lives at risk compared to fitting ATP, there have been very few fatalities since the fitting of AWS that would have been prevented had ATP been fitted instead. The Southall rail crash would not have been prevented by TPWS, but could have been prevented by ATP (ironically fitted but not in use), yet would almost certainly have been prevented had the AWS been working. A combination of TPWS and AWS is least effective in accidents like the one at Purley, where a driver repeatedly cancelled the AWS warning without applying the brakes, passing the danger signal at high speed. In this particular case though, the lower speed of the train and the very effective brakes of the EMU would have meant TPWS would likely have been effective anyway. Supporters of TPWS claim that even where it could not prevent accidents due to SPADs, it would likely reduce the impact, and reduce or eliminate fatalities, by at least slowing the train down. However, it is likely that in those cases the driver would have applied the emergency brakes well before the overspeed sensor. Both AWS and TPWS failed to prevent the 2021 Salisbury rail crash, because although the train went to full emergency braking, the slick conditions produced wheel slide and the train therefore was not brought to a stop (ATP would not have prevented this circumstance either). Locations in use The