Why are the US highway signs green?

A set of green route destination signs over a California highway

Photo: Gabriel Bouys/AFP (Getty Images)

Motorway signs is a ubiquitous part of America’s infrastructure iconography. The American highway shields are probably the most well-known mass-produced piece of aluminum in the country. However, the countless green signs that point every thoroughfare is a subtle basis for millions of journeys, from daily commutes to holiday road trips. These provide all necessary information to navigate the system, such as the upcoming exits and mileage to additional winding destinations. The design of these signs has also remained largely the same since the creation of the Interstate Highway System. But why was green chosen as the official colour?

According to Arizona Department of Transportation, green is used because it is a “bold” color. The green background of the sign tends to blend in with the greens, blues and browns of the natural landscape, while providing a nice contrasting surface for white text. John LaBarbera, an ADOT public information officer, stated, “It blends in enough to be considered part of the landscape, but sticks out enough to be noticed when you need it.” This explanation from ADOT covers the intuitive thinking behind the choice of color.

The standard for green for signboards originates from the manual on uniform traffic devices (MUTCD). The first edition of the MUTCD was published in 1935 by the American Association of State Highway Officials (today’s AASHTO), a standards body composed of representatives from all state departments of transportation. The initial manual was primarily focused on road markings, warning signs with a black-on-yellow background and regulatory signs in a black-on-white background across the country. There was no standard for guide signs as long distance driving was still not as common as we find it today. Travelers were expected to use route markers and their own maps.

Direction signs were officially standardized as white-on-green background signs in 1954, two years before the passage of the Interstate Highway Act. This significant change was included in a 15-page supplement to the 1948 edition of the MUTCD. This supplement also stipulates that stop signs must be white text on a red background. Before this change, stop signs were allowed to be either black or red text on a yellow background, in line with the other warning signs.

Direction signs were officially standardized as white-on-green background signs in 1954, two years before the passage of the Interstate Highway Act. This significant change was included in a 15-page supplement to the 1948 edition of the MUTCD. This supplement also stipulates that stop signs must be white text on a red background. Before this change, stop signs were allowed to be either black or red text on a yellow background, in line with the other warning signs. AASHO avoided red plates in the 1930s because fade-resistant red paint finishes did not yet exist.

A recreation of a vintage black text on yellow background stop sign.

Photo: artistmac / flickr

The current 2009 edition of the the manual on uniform traffic devices specifies the standard for color signs in section 2D.03.02:

“Except as otherwise provided in this manual for individual signs or groups of signs, directional signs on streets and highways shall have a white message and border on a green background. All messages, borders and explanations shall be retroreflective, and all backgrounds shall be retroreflective or illuminated.”

Without this standard, the US could have ended up with a kaleidoscope of character colors. Arizona once even experimented with color-coded signs based on direction. Blue for westbound signs, brown for eastbound signs, orange for northbound signs and green for southbound signs.

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