> They speak of this as if it was a Japan-only thing.
Which other countries have blue traffic lights?
Which other countries have blue traffic lights?
綠 is used in Japanese for the color of green tea, but they use 青 for the green light
There's also 翠 in Japanese to mean a bright green, which in Chinese also has a blue connotation (based on the color of the kingfisher who dives 淬 to catch fish)
But in Korea they have the same confusion:
https://www.dreamstime.com/traffic-lights-road-blue-speed-li...
So using Chinese as an example of "another language" having green-blue confusion isn't very meaningful since modern Chinese aka Mandarin also inherits pretty much the same 青 albeit popular usage differs somewhat.
Yet the fact that there's no blue traffic light in China just hinges on the fact that people don't commonly refer to the "green" in traffic lights as 青 in China (but you could). The word 青 is used to refer to a broad spectrum of green and blue in different contexts, just happens people generally refer to the traffic lights as "綠" instead. And since there's no ambiguity to "綠" there's no confusion of whether the traffic lights should be "green" or "blue", and thus you don't see blue traffic in China (hopefully).
What would the evidence for this be? It's clear that the word "ao" does not derive from the Chinese pronunciation of 青. But its meaning could easily have shifted under the influence of the spelling. Do we have a source that writes the word without spelling it 青?