the top speed of the train and distance from the end of LGV route. Trains at high-speed take several kilometres to stop. Since trains will require more than one signal block to slow down, drivers are alerted to reduce speed gradually, several blocks before any required stop. The signalling system is permissive; the driver of a train is permitted to proceed into an occupied block section without first obtaining authorisation. Speed in this situation is limited to 30 kilometres per hour (19 mph) and if speed exceeds 35 km/h (22 mph), the emergency brake is applied and the train stops. If the board marking the entrance to the block section is accompanied by a sign marked Nf, the block section is not permissive, and the driver must obtain authorisation from the Signalling and Control Centre () before entering the block. Once a route is set, or the signalling centre has provided authorisation, a white lamp below the marker board is lit to inform the driver. The driver then acknowledges the authorization using a button on the train's control panel. This temporarily overrides the emergency braking system, which would otherwise stop the train when proceeding past the non-permissive marker's ground loop. When trains enter or leave the LGV from , they pass over a ground loop which automatically switches the driver's dashboard indicators to the appropriate signalling system. For example, a train leaving the LGV onto a French ligne classique would have its TVM signalling system deactivated and its traditional KVB (, English: beacon speed control) system enabled. Electronics It is one of the more advanced railway signalling systems in the world, although the implementation itself is still based on relays rather than solid state electronics. There are two components to the TVM-430 system: one ground-based, the other on board the train. Both run using Motorola 68020 class processors, and are programmed in Ada, a computer language often used in safety critical systems. The system makes extensive use of redundancy; the mean time between dangerous failures is estimated to be over 1 million years. The ground-based segment of TVM-430 resides in trackside boxes, which control stretches of track about 15 km (9.3 mi) long. Each one is linked to the line's centralized traffic control centre, and directly controls about ten blocks of track, each with its own track circuit. Signaling information is encoded in AC signals which are fed into the