Some traffic signals have several (I’ve seen up to 6) lights for different directions arranged in a vertical column, I’d guess this is what your source was referring to. For some reason these are really hard to find on google images, but here’s one.
Some traffic signals have several (I’ve seen up to 6) lights for different directions arranged in a vertical column, I’d guess this is what your source was referring to. For some reason these are really hard to find on google images, but here’s one.