Which of these is NOT commonly indicated by a triangular sign?

The reason why "speed limit ahead" is the correct answer to this question is that triangular signs are typically used to indicate warning or cautionary messages relating to road safety. Common examples of cautionary signs include those indicating "children crossing," "road narrows," and "tunnel ahead," as these signs alert drivers to potential hazards or changes in the road conditions ahead.

On the other hand, "speed limit ahead" is generally indicated by a rectangular or square sign, which is used to convey regulatory information such as speed limits or other rules of the road. This distinction in the shape of the signage is important for drivers, as it helps them quickly interpret the intended message.

Recognizing the specific meanings attributed to different sign shapes is crucial for maintaining safety and compliance while driving.