The trains are essentially modified TGV sets,[175][176] and can operate at up to 300 kilometres per hour (186 mph) on high-speed lines, and 160 kilometres per hour (100 mph) in the Channel Tunnel.[3][4] It is possible to exceed the 300 km/h speed limit, but only with special permission from the safety authorities in the respective country.[177] Speed limits in the Channel Tunnel are dictated by air-resistance, energy (heat) dissipation and the need to be used with other, slower trains.