Thought you guys might like to see the inside of a traffic cabinet! While it is not a PLC controller exactly these things are run by rugged unix boxes running the traffic control software. I used to work on these things all of the time so I thought I would share a bit.
In this picture you can see the controller in the middle. It is also from Siemens. Siemens just spun off their controller company into Yunex but you will still find a lot of traffic controllers out there made from the Siemens era - namely the M50 or M60. You can see it has a big DB15 cable coming out the side which is SDLC which is a serial protocol that controls the switching. It goes to the panel at the bottom which has solid state relays to control the actual signals. At the top you see a rack of detector cards. These get inputs when you go over loops of wire in the road and tell the controller there is someone that wants to go in that direction. Those also communicate over the SDLC bus. The tall rectangular thing next to the controller is a MMU which stands for multi monitor unit. These take all of the RYG signals from the lights and compare them on a physically hardwired card to make sure there are no conflicts (one opposite directions) green at the same time and if it sees that it will throw the whole cabinet into flash.
Also at the top you can see a rugged switch and a cellular modem connected to the controller over ethernet. Most of these signals are NOT connected to the internet at all but use private cellular or fiber. Usually they run a web page or a telnet interface you can use to remotely program them or run diagnostics. When we did things we bought cellular modems and used something like Pangolin.net to connect into the controller remotely to manage them. There is also ATMS software that can talk to many at a time and run advanced control algorithms but we did not have that at our shop.
Happy to answer any questions! :}