Most mandatory road signs are circular
because they are mandatory signs
Warning signs are typically yellow with black lettering or symbols to indicate caution. However, some warning signs can be white, especially in specific contexts, such as regulatory signs that convey mandatory actions or information. It's important to recognize the color and shape of each sign to understand its meaning. Overall, yellow is the most common color for warning signs on the road.
Mandatory symbols
WHAT icons? If you are referring to the SHAPE of the signs themselves, the shape of the sign itself signifies the type of information the sign conveys. (e.g.: Octagaonal signs are ALWAYS stop signs - Diamond-shape signs ALWAYS convey some type of warning message - etc).
a triangle
I'm pretty positive Diamond shapes signs are warning signs.
weight, size, Shape, height, state, matter
White runway markings and yellow runway hold lines
Yes, stop signs are octagonal in shape.
circles
boobies
In North America, they're round.