From 1977, a portable AWS permanent magnet was fitted ahead of the warning board on the approach to temporary speed restrictions (TSRs). This was a recommendation of the inquiry into the derailment at Nuneaton on 6 June 1975, which occurred when the driver missed a TSR warning board due to its lights being extinguished. From 1990, AWS permanent magnets were installed immediately ahead of certain 'high risk' stop signals, as a SPAD mitigation measure. This additional AWS magnet was suppressed when the associated signal showed a 'proceed' aspect. Since the introduction of the Train Protection & Warning System (TPWS) it is no longer current practice to use AWS for this purpose. SPAD indicators were also used. Limitations AWS has only two states; clear and caution. It does not provide any information about what the hazard is. AWS is a warning system which is designed to be cancelled by the driver. It is possible for a driver on a crowded suburban railway to run all day on cautionary aspects. Continually cancelling warnings can lead to a state of conditioning, sometimes called the "double yellow zombie" effect, where the driver fails to take appropriate action when necessary. This has led to a number of fatal accidents. It is possible for AWS to develop a wrong-side failure whereby the driver is given no indication or a 'clear' indication instead of a 'caution'. Because of this the rulebook states that "AWS does not relieve the driver of the responsibility of observing and obeying lineside signals and indicators." Also, there is no automatic brake application when a stop signal is passed. The newer TPWS, which is installed at signals where there is a risk of conflicting movements, the approach to reductions of permissible speed of more than a third and buffer stops, overcomes this problem. Bi-directional operation Because the permanent magnet is located in the centre of the track, it operates in both directions. The permanent magnet can be suppressed by an electric coil of suitable strength. Where signals applying to opposing directions of travel on the same line are suitably positioned relative to each other (i.e. facing each other and about 400yds apart), common track equipment may be used, comprising an unsuppressed permanent magnet sandwiched between with both signals' electro-magnets. Other countries The BR AWS system is also used in: Northern Ireland Railways Hong Kong, MTR East Rail line (only used by intercity through