How can you tell if a road sign is a regulatory sign?

A regulatory sign is designed to inform drivers of specific laws and regulations that must be followed while on the road. The typical appearance of these signs is a white rectangle or square shape, which makes them easily identifiable. This standard shape and color scheme convey important instructions, such as speed limits and no parking zones, indicating legal requirements that drivers must adhere to in order to maintain safety and order on the road.

Other options describe features or characteristics of different types of signs. For instance, bright yellow colors are usually associated with warning signs, indicating caution about potential hazards rather than regulatory rules. Flashing lights are often found on warning signs or school zone signs, signaling drivers to be alert rather than demanding compliance with a specific regulation. Circular signs with a red border generally indicate prohibitive rules (like a stop sign), but the color and shape alone do not define them as regulatory in the context of this question. Thus, the white rectangular or square shape is the definitive characteristic of regulatory signs.