Editing Traffic light (section)

== Waterways and railways ==
The three-aspect standard is also used at [[List of locks and dams of the Upper Mississippi River|locks on the Upper Mississippi River]]. Red means that another vessel is passing through. Amber means that the lock chamber is being emptied or filled to match the level of the approaching vessel. After the gate opens, green means that the vessel may enter.

Railroad signals, for stopping trains in their own right of way, generally use the opposite positioning of the colours; that is, for signals above the driver's eyeline, green on top and red below is the [[UK railway signalling#Lens placement and alignment|standard placement]] of the signal colours on railroad tracks.  There are three reasons for this variation: there is no risk that railway signals will be masked by a tall vehicle between the driver and the signal; train speeds in fog are much higher than for road vehicles, so it is important that the most restrictive signal is closest to the driver's eyeline; and with railway signals often in exposed rural locations, there is a risk of any signal other than the bottom one being masked by snow building up on the hood of the signal below.