Re: NSA recommends...
The PLC controlling traffic signals doesn't need to be updated every five minutes. Its a machine. For newer intelligent signals it will accept inputs that alter the sequence or sequence timing but the basic operation is always the same, as are the mandated delays such as the minimum yellow phase time.
The problem with IoT protocols is that they're Web based so when code is updated it often wants to bring updates to libraries which in turn might need updating because of any unexpected problem. Its a sort of doom loop of code update froth where the actual purpose of the update is lost in the updates to the updates. In real life process control a machine or system is a tested entity that's intended to work for an extended period of time doing the same thing day in, day out.
(IoT itself seems to be designed for the convenience of vendors since it seems intended to tie units to vendor's cloud system in the hope that there will be recurring revenue from this. There are several standardized ways to manage and modify remote equipment that aren't tied to a vendor's cloud. Personally, I find interaction with IoT kit slow, klunky and unreliable -- its sort of thing that might be tolerable for a home thermostat (assuming it works) but is totally unacceptable for operating a chemical plant.)