Road signs they are a ubiquitous part of American infrastructure iconography. US highway shields are probably the best-known mass-produced piece of aluminum in the country. However, the innumerable green signs dotting every artery are a subtle staple for millions of journeys, from the daily commute to vacation road trips. These provide all the necessary information for system navigation, such as upcoming outings and mileage to further launched destinations. Additionally, the design of these signals has been largely the same since the creation of the Interstate Highway System. But why was green chosen as the official color?
According to Arizona Department of Transportation, green is used because it is a “cold” color. The green background of the sign tends to blend in with the greens, blues and browns of the natural landscape while providing a great contrasting surface for the white text. John LaBarbera, an ADOT public information officer, said: “It blends in enough to be considered part of the scenery, but sticks out enough to notice when you need it.” This explanation from ADOT covers the intuitive line of reasoning behind color choice.
The green standard for guide signs comes from the Handbook on Uniform Traffic Devices (MUTCD). The first edition of the MUTCD was published in 1935 by the American Association of State Highway Officials (now the AASHTO), a standards body composed of representatives from each state department of transportation. The initial manual focused primarily on road markings, black-on-yellow warning signs, and black-on-white regulatory signs throughout the country. There was no standard for guide signs as long distance road travel was not yet as commonplace as we find it today. Travelers were expected to use route markers and their own maps.
Guide signs have been officially standardized as white background signs on a green background in 1954, two years before the Interstate Highway Act was passed. This significant amendment was included in a 15-page supplement to the 1948 edition of the MUTCD. This supplement also mandated that stop signs be white text on a red background. Prior to this change, stop signs could be black or red text on a yellow background, in line with other warning signs.
Guidance signs were officially standardized as white background on green background signs in 1954, two years before the passage of the Interstate Highway Act. This significant amendment was included in a 15-page supplement to the 1948 edition of the MUTCD. This supplement also mandated that stop signs be white text on a red background. Prior to this change, stop signs could be black or red text on a yellow background, in line with other warning signs. AASHO avoided red markings in the 1930s because fade-resistant red paint finishes didn’t exist yet.
The current 2009 edition of the Handbook on Uniform Traffic Devices lists the standard for guide signal color in section 2D.03.02:
“Except as otherwise provided in this Manual for individual signs or groups of signs, guidance signs on roads and highways must have a white message and a border on a green background. All messages, borders and legends must be retro-reflective and all backgrounds must be retro-reflective or illuminated.
Without this standard, the United States could have ended up with a kaleidoscope of sign colors. Arizona once even experimented with direction-based color-coded signs. Blue for westbound signs, brown for eastbound signs, orange for northbound signs, and green for southbound signs.