Why are traffic lights red, yellow, and green?
The short answer
Traffic light colors are shared signals that tell people when to stop or when they may move. For Japan's round traffic lights that do not flash, red means stop. Green means you may move if it is safe. Yellow normally means stop.
Why colors help
Signals need to be easy to recognize from a distance and to understand without reading words. Using the same rules for colors and the order in which they light up helps drivers and people walking understand the same signals.
Yellow normally means stop
In Japan, a round yellow traffic light that does not flash means that vehicles must normally stop at the required stopping point. If there is a stop line (a line showing where vehicles must stop), stop just before it.
A vehicle may continue if it is already too close to that stopping point to stop safely when the light turns yellow. Yellow is not a signal to speed up.
A flashing yellow light has a different meaning
A yellow light that keeps turning on and off means you may move while watching carefully for other vehicles and people walking. Its rule is different from that of a yellow light that stays on.
The traffic rules explained here apply in Japan. In other countries, follow the local traffic rules.