What indicates that you must stop at a stop sign?

The red octagonal sign is universally recognized as the symbol for a stop sign. This shape and color combination is standardized in traffic signage, meaning that drivers must come to a complete stop when they encounter a stop sign. The significance of the octagonal shape is that it differentiates the stop sign from other types of traffic signs, ensuring that road users can easily and quickly recognize the need to stop.

Signs like the flashing yellow light or solid white line may indicate caution or guide stopping behavior but do not enforce a stop in the same manner that a stop sign does. Similarly, a yield sign indicates that you must give way to oncoming traffic, but it does not require a complete stop unless necessary for safety. Thus, the red octagonal sign clearly communicates the requirement to stop, making it the definitive indication for this action.