Traffic colour

the word that changed the lamp

Unicode: U+1F6A6 vertical traffic light · U+26D4 no entry — the signals themselves are hardware, not characters
The job: Command a driver travelling at speed, in poor light, with roughly one bit of available attention.
Standards: The Vienna Convention on Road Signs and Signals fixes the red / amber / green sequence across its signatories. The grammar has a second layer that is easy to miss: position encodes the command redundantly. Red is always at the top of a vertical signal, and at the outer end of a horizontal one, so that a driver who cannot distinguish the colours reads the geometry instead.
Failure modes: The redundancy exists because the primary channel was a poor choice. Red and green are precisely the pair that the commonest inherited colour-vision deficiencies confuse, affecting something like one man in twelve — and they were adopted anyway, because red was already the railway danger colour and the convention was inherited rather than designed. The system encodes its most important instruction in the one channel a large minority cannot read, then patches it with position. Amber compounds it: flashing amber carries different meanings across jurisdictions.
Cultural flips: Japanese signals are the clearest case anywhere of language shaping hardware. The statutory “go” colour is 青 ao, a term that traditionally spans blue and green, and Japanese signals are manufactured at a noticeably bluer end of the range as a result. The word did not describe the lamp; the word changed what the lamp had to be.

Sources · 3

documented Vienna Convention on Road Signs and Signals, 1968, for the colour sequence and signal conventions.
documented The Japanese use of 青 for the permissive signal, and the resulting blue-shifted lamps.
documented The prevalence of red–green colour-vision deficiency, and position redundancy as the standard mitigation.