before the automatic emergency brakes come on. Overspeed A small amount of over-speed allowance is made before the train's emergency brakes are applied. At speeds below 80 km/h, the allowance is five kilometres per hour. At nominal speeds of 80–160 km/h, the tolerance is ten kilometres per hour and at speeds in excess of 160 km/h, it is fifteen kilometres per hour. Cab display In the centre of the driver's desk in a TGV cab, just below the windscreen, there is a double or triple row of square indicators. This is where target speeds for the current and subsequent blocks are displayed to the driver, in the form of numbers (in kilometres per hour) on a colour-coded background. Full line speed is indicated in black numerals on a green background, while slower aspects are indicated in white numerals on a black background and a full stop is indicated as "000" on a red background. Below this display is the speedometer, where the continuously varying target speed is indicated as well as the current speed. Speed is measured by the train's redundant tachometers to a precision of ±2%. The allowable variation between target speed and actual speed is dependent on speed. All the in-cab signalling displays must be very reliable, since they are critical to safety. They have relay-based position sensors which feed back to the signalling computer the current aspect being displayed to the driver. If there is a failure in the display unit, appropriate action is taken to stop the train. In order to reduce stress on the driver, speeds are displayed over several blocks ahead of the train. When a block is followed by a more restrictive (slower) block, the display for that block flashes so the driver can better anticipate the speed change without releasing the brake. Restrictive indications can only be updated at block boundaries, except in emergencies. They are accompanied by an audible in-cab horn signal. Restrictions can however be lifted at any time within a block. TVM-430 has extensive redundancy built into it, and one might wonder why it is not used to control the train directly. However, in view of the lack of adaptability of the system to unexpected situations, it is considered desirable to retain a human in the loop. Driving a TGV is therefore done entirely manually, but the signalling system keeps a very close watch to ensure maximum safety. Other