Why is red used for stop signs and danger signals?

The use of red for stop signs and other danger indicators dates back to 1841, when British railroad magnate Henry Booth lobbied to standardize the color scheme for all rail yards. In the original schema, green meant "caution" and white meant "go."

The red-green-white arrangement existed until after an accident decades later, when a red lens fell off a signal light, giving the train a false white "go" signal. After that, officials altered the scheme so that green meant "go," avoiding conditions that initially caused the accident. Automobile traffic control architects copied this scheme during the implementation of road controls after the development of cars.