Why does green mean 'go' at traffic lights, instead of white?

Answer: Railways dropped white after broken red lenses showed white

Green light carries farther than white light — Green light doesn't carry farther than white; white light actually contains green along with every other color. The switch was about safety: white could appear by accident, green couldn't.

Railways dropped white after broken red lenses showed white ✓ — Early railway signals used white for 'all clear'. But when a red lens cracked or fell out, the bare lamp shone white and a driver read 'go', so railways switched 'clear' to green in the late 1800s, and road traffic lights later copied the railway colors.

Early laws chose green because it calms drivers — No early law picked green to calm drivers; the colors were inherited from railway signals. Green replaced white there because a broken red lens could never be mistaken for green.