What type of sign is typically used to indicate a warning?

The diamond-shaped sign is specifically designed to indicate warnings to drivers. This shape is universally recognized for alerting motorists to potential hazards ahead, such as curves, intersections, or road work. The use of a diamond shape helps in quickly drawing attention to warnings, as it is distinct from other sign shapes associated with regulations or guidance.

In contrast, square and rectangular signs are typically used for regulatory or informational purposes rather than for warnings. Circular signs generally indicate rail road crossings or specific types of alerts but are less common than the diamond shape for warning signs. Thus, the diamond shape is the most effective and widely used for this purpose, making it the correct choice for indicating warnings on the road.