In the United States, most warning road signs under the MUTCD are diamond-shaped.
The Manual on Uniform Traffic Control Devices, usually called the MUTCD, sets national standards for traffic signs, signals, and pavement markings on roads open to public travel. Standard U.S. warning signs are generally yellow with black symbols or words and are mounted as diamonds, meaning a square turned on a point.
The diamond distinguishes a warning from a regulatory sign, such as a speed-limit sign, and from a guide sign, which is commonly rectangular. Examples include curves, merging traffic, intersections, pedestrian crossings, and animal crossings.
Some warning signs use special formats or supplemental plaques, so “most” is more accurate than “all.” The shape and color system lets drivers identify the type of information quickly, even before they can read the symbol or wording.