For automatic block signalling (ABS), the headway is measured in minutes, and calculated from the time from the passage of a train to when the signalling system returns to full clear (proceed). It is not normally measured tip to tip. An ABS system divides the track into blocks, into which only one train can enter at a time. Commonly trains are kept two to three block sections apart, depending on how the signalling system is designed, and so the size of the block will often determine the headway.