A stop sign is coming that you cannot see yet. The warning gives you the braking distance the hidden sign cannot.
Begin slowing now. By the time the real octagon appears, you should be able to stop behind its line calmly, not dramatically.
A small stop sign inside a yellow diamond: the preview of the octagon.
Waiting to see the actual stop sign before braking. On a 55 mph road, that is often too late to stop smoothly, or at all.
Signs that live in the same family, or get confused with this one.
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