The local councils (local Authority) usually get external companies to design the systems, but there are standard, off the shelf solutions that are specified to be used. These systems range from time synced junctions (with Vehicle attenuation) up to full Urban Traffic Control systems, the protocols used are mostly standard (UTMC), so the local Authority just needs to say what they need. For a UTC system it used to be that the council would run the instation, but with cloud computing, that's mostly run for them now as a hosted system. Usually this is all controlled by the standard local council procurement rules, and run by council employees not elected officials. The elected officials can say a scheme is needed and approve costs, but not who does it.