This is pretty far off base - for a start, the first JP law on traffic lights in the 1930s used midori, not ao. Per jp references, ao/midori split into distinct meanings around the Heian~Kamakura periods (so 800-1000 years ago).
Have a source for that? This section[1] of the ja.wikipedia article could apparently use your revisions - both kanji are acceptable.
[1] https://ja.wikipedia.org/wiki/%E4%BA%A4%E9%80%9A%E4%BF%A1%E5...
If you want a source that the 1930 law used midori, see here, under 発祥期:
https://ja.m.wikipedia.org/wiki/%E6%97%A5%E6%9C%AC%E3%81%AE%...
In 1930, not since the 1930s. The 1930 law used midori, but it was changed to ao in 1947 after that became the more popular usage. The page you linked was about the current law.