What type of sign indicates "road curves ahead"?

A yellow diamond sign is used to indicate caution and warn drivers of potential hazards ahead, including road curves. This specific shape and color combination is universally recognized for signaling drivers to slow down and prepare for changes in the road's direction. The diamond shape is particularly effective because it stands out to drivers and conveys that they need to pay attention to the upcoming road conditions.

In contrast, a green rectangular sign typically provides information about directions or facilities, while a white circular sign generally conveys regulatory information, such as speed limits. A red triangular sign is used for warning signs, such as yield signs, which instruct drivers to yield the right of way. Thus, the yellow diamond sign specifically communicates the necessary caution for upcoming curves in the road, making it the most appropriate choice.