Even if you can't explain the significance behind every color and symbol used in road signs, you may understand them on a subconscious level. That's why the designs are chosen in the first place: Our brains associate colors with certain feelings, and on the road, a symbol's ability to communicate danger in less time than it takes to read a word could mean the difference between life and death.
This was taken into consideration when the federal government standardized the markings used to separate traffic lanes in 1971. Today the lines are painted in two colors: White for when both traffic lanes are traveling in the same direction and yellow for when they're not. The distinction is meant to prevent accidents, but it took years to convince officials that it was the right choice.
Edward Hines designed the first modern centerline for a road in the early 1900s. He made it white, inspired by spilled milk he once saw on a freshly paved road, and that color remained the default for decades. By 1955, most states used white stripes to divide their traffic lanes. The one exception was Oregon. The state insisted that yellow was a better way to signal caution—a claim the rest of the country didn't buy. Oregon ultimately agreed to change its centerlines to white when the government threatened to withhold $300 million in highway funding.
By 1971, the people in charge of standardizing highway symbols had come around to Oregon's point of view. The case for yellow as the color for caution was stronger than ever: It had been implemented in stoplights as the signal for slow and it was even the color of stop signs in the early 20th century.
But not every centerline needed to come with such a strong warning. While the color of lines separating parallel traffic flow remained white, yellow was used as a buffer between cars driving in the opposite directions—in other words, the lines that are most dangerous to cross. That rule in the 1971 edition of the Manual of Uniform Control Devices for Streets and Highways is still standard today.
Many road sign features, like the green in interstate signs, have interesting origin stories. Here are more facts about the roads you take every day.
[h/t Reader's Digest]