Reply #9: In the early 70's, Columbia, MD had horizontal traffic lights - great unless you're color blind!

[HopeHoops]

There was no way to tell which side was green or red if they look the same to you. That caused enough accidents that they went back to traditional vertical lights. A lot more people are color blind than most people realize. The majority can't distinguish between green and red. Blue/yellow is far less common. I read an explanation for that many years ago but I've long since forgotten what it was.