The system currently uses automatic block signaling with fixed wayside signals and automatic train stops in order to provide safe train operation across the whole system. The New York City Subway system has, for the most part, used block signalling since its first line opened, and many portions of the current signaling system were installed between the 1930s and 1960s. These signals work by preventing trains from entering a "block" occupied by another train. Typically, the blocks are long. Red and green lights show whether a block is occupied or vacant. The train's maximum speed will depend on how many blocks are open in front of it. The signals do not register a train's speed, nor where in the block the train is located.