Traffic lights
Traffic lights have THREE colours, and each is an instruction.
RED: stop, behind the stop line. GREEN: go, if the way ahead is clear. YELLOW/AMBER: prepare - stop if you can do so safely, and clear the junction if you are already in it.
An automatic traffic signal is the electric light installed at the road; the other kind of signal is the one given by a traffic officer. Where a police officer is directing traffic, the officer overrides the lights.
Green does not mean the junction is clear. Look before you move.