Railroad signals communicate track conditions to the engineer of an approaching train and are vital to rail safety, because without signals it would be impossible to run multiple trains on a line. Though some resemble traffic lights, railroad signals are more complicated, and each railroad is responsible for interpreting its own signaling system, which have historically varied widely. At their simplest, signals might convey proceed, proceed with caution, or stop, while some can communicate instructions as complex as proceeding prepared to stop before passing the next signal and requiring trains exceeding 35 mph to reduce speed immediately. One of the first signal types was a simple ball on a vertical pole and pulley, manually positioned higher or lower, and as the railway system grew, signal technology evolved.