The warning sign is usually which shape?

The shape of a warning sign is typically a diamond. This unique shape is designed to alert drivers to potential hazards or changes in road conditions ahead. The diamond shape is easily recognizable and distinguishes these signs from regulatory signs or guide signs, which may have different shapes. The use of a consistent shape for warning signs helps drivers quickly understand the nature of the warning and encourages them to adjust their speed or take caution, thereby enhancing road safety.

Other shapes mentioned in the options serve different purposes: an octagon specifically denotes a stop sign, while a pentagon is used for school zone signs, and a rectangle often designates regulatory signs or guide signs. Understanding these shapes is crucial for recognizing road signs and ensuring safe driving practices.