What does a yellow traffic light warn?

A yellow traffic light warns of a change in signal status and tells road users to prepare to stop when it is safe. “Caution” is the canonical short quiz answer, although the exact legal rule varies by jurisdiction.

In the usual sequence, yellow appears after green and before red. In the United States, a steady circular yellow means the green movement is ending and a red signal will soon appear. Under international Vienna Convention guidance, traffic should not cross the stop line on amber unless stopping safely is impossible.

Yellow does not generally mean “speed up.” Accelerating to beat the light can create a dangerous dilemma zone, especially for vehicles close to the intersection. Nor does it always mean an immediate stop: a driver who is already too close to brake safely may continue through. Flashing yellow is different again; it commonly permits cautious entry while requiring attention to other road users.