safety
circles
Yes, stop signs are octagonal in shape.
yes
A large X, referred to as a crossbuck.
The "No Passing Zone" sign is a triangular "pennant" shape, pointing to the right side. It is either yellow or orange in color.
The color of a recreational sign is typically green with white lettering.
safety
Warning signs are usually a diamond shape. The color of the sign is yellow in order to caution drivers about an intersection ahead, bicyclists or wildlife that may be near the road.
Three signs of physical change are change in shape, change in size, and change in color.
A regulation sign is a sign that gives you the regulation for something. These signs are always in white and black and rectangle in shape. An example is a speed limit sign.
The cusp
An orange triangle sign indicates a slow-moving vehicle on the road.
Spain uses International road signs. The shape, pictograph and color of the sign indicates what is permitted and what is not. They also use Stop (English) and Ceda el Paso (yield in Spanish) even though the sign shape itself is sufficient.
Signs intended to warn you of a hazard may be called "safety signs" by some, and they are red and yellow in color. Safety signs that are intended to tell you that something is "safe" are green in color.
Most mandatory road signs are circular
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).