pair of loops is located at the signal, and will activate the emergency brake if the train passes over them at any speed when the signal is at danger. TPWS has proved to be an effective system in the UK, and has prevented several significant collisions. However, its deployment is not universal; only those signals where the risk of collision is considered to be significant are fitted with it. Flank protection At certain junctions, especially where if the signal protecting the junction was passed at danger a side collision is likely to result, then flank protection may be used. Derailers and/or facing points beyond the signal protecting the junction will be set in such a position to allow a safe overlap if the signal was passed without authority. This effectively removes the chance of a side-impact collision as the train would be diverted in a parallel path to the approaching train. SPAD indicators Prior to the introduction of TPWS in the UK, "SPAD indicators" were introduced at 'high risk' locations (for example: the entry to a single track section of line). Consisting of three red lamps, they are placed beyond the protecting stop signal and are normally unlit. If a driver passes the signal at 'danger', the top and bottom lamps flash red and the centre lamp is lit continuously. Whenever a SPAD indicator activates, all drivers who observe it are required to stop immediately, even if they can see that the signal pertaining to their own train is showing a proceed aspect. Since the introduction of TPWS, provision of new SPAD indicators has become less common. UK acronyms: SPAD / SPAR In the UK, incidents where a signal is passed at danger without authority are categorised according to principal cause. A SPAD is where the train proceeds beyond its authorised movement to an unauthorised movement. Other types are categorised as SPAR ("signal passed at red"). Prior to December 2012, the term "SPAD" applied to all such incidents, with a letter specifying cause. A SPAD (formerly Category A SPAD) is where the train proceeds beyond its authorised movement to an unauthorised movement. A Technical SPAR (formerly Category B SPAD) is where the signal reverted to danger in front of the train due to an equipment failure or signaller error and the train was unable to stop before passing the signal. A Signaller SPAR (formerly Category C SPAD) is where the