Yellow is used for cautionary traffic signs because of its maximum visibility, day or night. State and local governments must follow the national standards and laws for traffic-control devices, including sign colors, or risk legal action or loss of federal funding, per the Manual on Uniform Traffic Control Devices.
Cautionary traffic signs also have a standard diamond shape. Stop signs were yellow from the late 1920s until 1954 when a reflective aluminum material became available and stop signs were required to be red, according to Road Traffic Signs. Colors and shapes are used to classify all traffic signage. Black and white are used for regulatory signs, such as speed limits and passing zones. Orange signs signify construction areas, and brown signs identify recreation areas, scenic overlooks and byways.
Traffic signage was first addressed in 1899 by horseless carriage owners who gathered to place and maintain roadside directional signs, according to MUTCD. Detroit posted the first official street sign, according to The New York Times. Furthermore, the first steps toward uniformity across states were taken in the 1920s when representatives from Wisconsin, Minnesota and Indiana toured other states, hoping to create a foundation for uniformity of traffic signs and road markers.