Sometimes here they will add an additional section for a bi-color protected turn. i've never seen a doghouse or 5-section signal here. All protected turns are bi-color and the big intersections have a separete R-Y-G arrow signal.
Over here when they add an arrow to a signal they usually change the head, I think they do that because our signal heads have backplanes on them which makes it harder to just add a section.
I've seen them add sections on to pedestrian signals when they retrofitted countdowns on them though.
The older ones here don't have the backplates on them. There was a newer install that got new section and it had a backplate. They just removed the backplate and added the section. Oddly, the only two signals at the intersection was that that one and the one next to it facing the same direction.
In PA, traffic signals with backplates weren't very common the 80's, but in the mid 90's backplates became common on new traffic signal installations. The 5 section doghouses in my area started showing up in 1989.
@ Joe; yep, it's been a recent thing. I know of one intersection that has always had backplates since before I was born. the black paint on them is virtually gone. That's the only one i know of that used backplates before 2008. I think here they're used mainly as a decorative thing. The Signals are yellow with black backplates and galvanized steel poles and masts. In downtown areas, the signals, backplates, poles, and masts are all black.
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I've seen them add sections on to pedestrian signals when they retrofitted countdowns on them though.