This is fascinating because I have never given this much thought! I am in Canada, and I believe I have used both -- perhaps not sure if there even was a correct way to say it! After doing a quick verbal test...it seems I do say "traffic lights". Maybe it's because the intersection has more than one light? Perhaps because there is more than one light within each unit? It may be regional after all!

Yes, in the US, a set of red yellow and green lights makes up one traffic light. In fact, we usually conceptualize it as one light that changes color from green, to yellow, to red, to green again. We often speak of “waiting for the light to change”. We don’t talk about the red light turning off and the green light turning on. So in theory, it’s really just one light that changes colors. In the past, that wasn’t technologically viable, but nowadays, isn’t it high time they installed color-changing LED units? Note: We can also refer to the “(traffic) signal”. I believe they may say “semaphore” (which to me would be a different type of signal) in the UK.

'Yes' to 'traffic lights' being plural in the UK, Nikola (but I'm sure you know that). We "wait for the lights to change" (all three of them, doing their thing). Plural sounds most natural to me, but I wouldn't be surprised to hear younger people saying 'traffic light'. American influence is all-pervasive. 'No' to 'semaphore', Phil. That's flag-waving. Curious to know where you heard this misinformation, though. And the single-light idea wouldn't work here, as we have red and amber together before the green light. And I'm looking forward to hearing our South African friends confirming that they say 'Robot' down there!

Yes, in the U.S., people say "And then you will come up to a traffic light, and when you do, make a right turn..." So the whole object (with three lights) is called "a traffic light" (singular)