The colors derive from maritime and railroad customs.
For generations, ships have used red, green, and white lanterns to designate privileged and burdened transit privileges. A green light indicates that the ship is privileged, whereas a red light indicates that the ship is burdened and must turn right or slow down. If both red and green lights are visible, ships are approaching each other head-on and must turn right. The white light shows that the vessel is passing another vessel traveling in the same direction. Multiple white lights signify that the ship is powered by the wind, which is always favored by overpowered vessels. Multiple red lights signal that the ship is too large to rapidly alter its direction or speed.
When railways first began utilizing colored signals, red meant stop, green meant caution, and white meant to go. These hues were acceptable for signal flags. However, white was problematic at night, when engine drivers confused other lights for the signal lights. Thus, the colors indicating stop, caution, and go were altered to red, yellow, and green respectively. However, railroad warning signs indicate that the next light will be red since the stopping distance of a train is greater than the visibility distance. The car warning signal gives drivers sufficient time to halt for a subsequent signal.
Ships and aircraft utilize the same color designations.