Why Are Highway Signs Green? - Jalopnik

Guide signs were officially standardized as white-on-green background signs in 1954, two years before the passage of the Interstate Highway Act.

Prior to this change, stop signs were allowed to be either black or red text on a yellow background, in line with the other warning signs.

Guide signs were officially standardized as white-on-green background signs in 1954, two years before the passage of the Interstate Highway Act.

This supplement also mandated that stop signs be white text on a red background.

Prior to this change, stop signs were allowed to be either black or red text on a yellow background, in line with the other warning signs.

“Except where otherwise provided in this Manual for individual signs or groups of signs, guide signs on streets and highways shall have a white message and border on a green background.