Corbis
There were only red and green lights on the semaphore column--instead of a main yellow light, red would come on in all four directions during the change of semaphores, giving time for everyone to stop before the green light and "GO" semaphore came on for the new direction of movement. I think I read that the semaphores were turned off at night, with small flashing lights (under the main ones) coming on, yellow for the main drag, red for the secondary street. Anyway, I guess the stop sign was for extra safety in case of signal malfunction. They don't seem to be on all, or even very many, semaphore poles in vintage pictures I've seen, even when the signals were new. Maybe the combination only lasted a short time because of confusion.