They measured 40 centimeters in diameter,and they were red,amber,and green.However, at the time,red meant traffic in all directions had to stop,amber stopped only east-west traffic,and green stopped only north-south traffic.This was very confusing,so in 1924 the city adopted the system used in railroads: red for stop,green for go,and yello for slow.This system because standard all over the world.