Why Does Red Mean Stop and Green Mean Go?

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In the 19th century, rail yards all used their own signal systems. Then, in 1841, railway bigwig Henry Booth pushed to standardize Britain’s color scheme, suggesting red signal “stop,” white signal “go,” and green signal “caution.” Decades later, after a train plowed through a stop signal when a red lens popped off, causing a fake white “go” light and a nasty crash, officials decided to eliminate confusion by giving green an upgrade.

Traffic lights, which were just appearing on American roads, piggybacked on the new color scheme—with a few exceptions. In New York City until 1925, drivers going east or west stopped at an amber light and drove on green, while drivers going north or south drove on amber and stopped at green. Tourists loved it!

This story originally appeared in mental_floss magazine. Subscribe to our print edition here, and our iPad edition here.