When was it first established that Green was safe and Red was danger?

Green and red were first used on ships as navigational lights (red = port; green = starboard). These colours were chosen after much experiment as those which could be best distinguised over long distances.

When navigation lights were required by the railways (and, later, the traffic lights) it was natural to borrow the green and red system from the maritime world. I don't know who decided that of the two, red meant stop and green meant go - but after this it was a natural step for the meaning of red to extent from "Stop" to "Danger."

Not sure but they couldn't have picked a worse colour combo as green and red is the one which most colourblind people are unable to distinguish between