A stop sign is a traffic sign that is used worldwide to indicate that drivers are required to come to a complete stop before proceeding further. The sign is typically an octagon with the word "STOP" in white letters on a red background. The reference to "Red alert?" suggests that the color red is associated with alertness and caution in this context.
Note: The provided meaning of the answer is for the latest New York Times crossword puzzle clue: Red alert?. Explanations are generated by AI and may be inaccurate.