Which is why (pause as I get on my ludicrously rickety Oklahoma-vs-Texas soap box once again) horizontal lights are dumb, because Britain drives on the left too, and their vertical lights are the same as ours after they're mirrored! 
Wouldn't mirrored vertical signals always look the same? Unless we were driving upside down (aka only applicable in Australia)?
That's what I'm saying—if you mount the signals vertically, there will be no need to mirror them in left-hand drive countries, because then they're horizontally symmetrical.
(I also don't know that the left-hand-drive aspect is why the red signals are on the right there. It could also be because of Japanese reading order.)
Sorry, missed the emoji the first time around!
Still, being curious, I've looked around at other parts of the world that drive on the left to see if any even use horizontal traffic signals.
Overall, they are very rare. Japan probably has 99.9% of them. But I did find one in USVI, and it displays the green and red opposite what I see in Japan:
(image from
https://resilientvi.org/)
I've long understood traffic lights in Japan to be opposite what you'd see in the US because, well, they drive on the left, so everything is the opposite. But visually, they're following the same rule that I think is used in the US, whereby the horizontal signal is moved vertically first, rotated counter-clockwise 90 degrees, and then moved out onto the mast arm. In Japan, the signal is moved vertically first, rotated
clockwise 90 degrees, and then moved out onto the mast arm...in both cases, the red signal is kept in the "leading" or "outer" position.
Obviously this isn't actually how signals are physically placed, but I always thought the rule was to keep red either in the most vertical or most outer position, at least for signals on the driving side of the street (signals in Japan are often mounted on the far left and near right side, but the layout is always red on right, green on left).
