Traffic signals come from "signals" used by railways. The earliest train tracks were laid on carriage roads in England. British law requires locomotives to be accompanied by a flag-bearer who walks in front of the train to warn other vehicles and horses, for which the flag-bearer must carry red flags and red lanterns. Red is the color associated with "danger." (This may have something to do with red being the color of blood.) In the UK, after rails were removed from roads and changed to different lines, railways developed signaling systems that gave train engineers instructions to stop, drive, or drive carefully. The original roadway traffic signals usually used only two colors, red and green, although some cities used green for east-west traffic, yellow for north-south traffic, and so on. This was obviously confusing, and eventually, there was a consensus that red would mean stop and green would mean going, regardless of the direction of travel. Yellow has been associated with "caution" through its use on warning signs when a third shot was added to traffic signals to indicate changing intervals.