Which is idiomatic in British and American English, when talking about a single post that contains 3 lights, red, yellow and green? A traffic light or A SET of traffic lights?
Dictionaries seem equivocal about this.
- The Longman Dictionary of Contemporary English lists only the plural noun traffic lights. This implies that each of the red, yellow and green lights are considered separately.
- The Oxford Advanced Learner's Dictionary lists a singular form traffic light, with a note that says "(also traffic lights [plural])", and a definition that says "a set of lights that controls the traffic on a road." Based on this definition, the singular form seems to be able to refer to all three lights together.
- The Cambridge Advanced Learner's Dictionary would beg to differ. Like the OALD, it also lists the singular form traffic light, but the definition says "one of a set of red, yellow, and green lights that control the movement of vehicles", meaning that a "light" really means a single light, not a set of three lights.
There have also been multiple spirited online discussions. Two Wordreference threads (1 and 2) seem to arrive at the conclusion that traffic light is idiomatic in the US, while traffic lights is in the Commonwealth. More users from italki seem to agree with this intuition, too.
If it is indeed the case that traffic light is American English and traffic lights is Commonwealth English, I guess the only thing left is confirmation from an authoritative source, preferably some sort of book or guideline from a transport(ation) authority.