[schappim]

TL;DR: The traditional Japanese language did not have separate words for green and blue; instead, it used a single term "青" (ao) to cover both colours. Over time, the language evolved, but the initial categorisation of green as a type of blue stuck around in certain contexts, like traffic signals. This linguistic trait led to the designation of traffic signals that are green in colour as "青信号" (ao shingou), which literally translates to "blue signal."