I live in the US where traffic lights in all of my town are vertical
(red on top, green on bottom) but in the next town (not even 5 minutes
away) most of their traffic lights are sideways; red on the left and
green on the right. Why do they have "sideways" traffic lights?

Hi, 
I called...
Traffic Maintenance
4303 S. 50th St.
Omaha Ne. 68117
402.444.5160
 .....here in Omaha NE, and this is what they had to say. 
1. Lights are sometime mounted sideways so that they are more easily
seen to drivers such as, after a underpass, or somewhere with low
visability. If they were vertical, you could only see part of the
light as you came up to it, but sideways, you can see all 3 lights at
once. It's based on safety.
2. Depending on what the lights are mounted on, sometimes sideways has
better weight managment, such as in a rural area or a windy part of
the city, allowing them to hang would cause damage from the swinging
that would happen. Sideways, you have more surface area mounted, esp.
if it is a wire mount, and it minimizes the movement of the light.
And last, but not least
3. Sometimes, in certain areas, the planners try and minimize the
appearance of the lights, for purely asthetic reasons. Same reasons
some neighborhoods have lanterns instead of streetlights. It's the
development temas input.

So, there really is no "law" governing the placement of lights, it's
not based on anything concrete, such as the weight of the light or
something mathematical formula deciding.... vertical or horizontal.
It's pretty much based on those 3 factors, be it safety, looks, or
wind.

Nenna-GA

thank you!