 Confronted with the fact that train drivers would not be able to see signals along the track-side when trains reach full speed, engineers developed the TVM cab-signalling technology, which would later be exported worldwide.It allows for a train engaging in an emergency braking to request within seconds all following trains to reduce their speed; if a driver does not react within 1.5 km (0.93 mi), the system overrides the controls and reduces the train's speed automatically.The TVM safety mechanism enables TGVs using the same line to depart every three minutes.