Here in Texas, all lights are mounted horizontally. I have been searching and searching and cannot find out why… But I work in media and I’m going to get in touch with TxDOT to find out and I’ll post back. Where I am in Waco, most all lights are horizontal on mast poles. Some of the smaller towns use wire but most are all on masts. In Dallas and Ft. Worth that’s also the case for the most part, except downtown. Houston has a crude mix of wire and mast mounting, but again all horizontal. The main reasons I’ve come up with why we mount them horizontal in Texas is 1) just to be different or 2) the mounting is more secure. When the lights are mounted on masts, the bracket holds the light at both ends. It might be the same way when the light is mounted on a mast vertical. In my opinion when they’re hung on a wire horizontally it looks rediculous, especially the way Houston does it. I think a lot of parts are of the city are like that because the roads are always being worked on. It seems like everywhere you go there’s always a lane closed, a pothole covered with a metal plate, etc. etc. so crews are always having to move the signals.
Way, Way back when I was first driving, 60 years ago, Atlantic City had strange lights. On the long streets (NE to SW) they had red on top and green on the bottom (no yellow, simultaneous green and red signaling the light was changing), while the cross streets had green on top and red on the bottom. The point is that each traffic signal contained only two bulbs that shone in all directions. You got used to it, but it was a problem for my color-blind father.
Someone said that Quebec used horizontal lights. Most of the lights in Montreal are vertical and few are shape coded. I don’t think it is that different out of the city. One useful innovation that should be more widely copied: a blinking green is equivalent to a left arrow. Very useful.
That would be the Darley Simplex traffic signals that had green on the top for one direction. They also usually had a simple electromechanical controller integrated so the agency would just have to hang it and string AC power to it. They turn up on eBay from time to time, north of $1000 if in decent condition.