Train movements in most rail systems are tightly controlled by railway signalling systems, or signalling block system. In many railways drivers are given instructions on speeds, and routes through the rail network. Trains (rollingstock) can only accelerate and decelerate relatively slowly, so stopping from anything but low speeds requires several hundred metres or even more. The track distance required to stop is often much longer than the range of the driver's vision. If the track ahead is obstructed, for example a train is at stop there, then the train behind it will probably see it far too late to avoid a collision.