IRT to applying ultra-high bandwidth networks to devices which don't need a lot of bandwidth, the point of moving to a higher bandwidth network is to allow these devices to quickly report back what they want to say and then get off the air faster. The faster they can report their small heartbeat of data, the more devices you can support. Lets assume this traffic light sends a snapshot of the intersection at a frame a second, each frame being 100KB. So, 100KB/s would be is normal load, so there's no real need for it to have a dedicated 100Mbit link. However, if you've got a few thousand devices wanting to share that link space, you're going to need more bandwidth available to the whole. Its also good for the general responsiveness of the network for that 100KB transfer to happen in less time, as only a single device can talk on the network at once. Its the same thing with home WiFi connectivity. 54Mbit would theoretically be plenty of bandwidth for a movie stream and a few phones/tablets browsing social media, but if you're using 802.11g you'll probably have issues with your video stream as those phones and tablets hog the airwaves while they load a batch of images. It takes time servicing multiple devices in the same channel, time that takes away from max effective bandwidth available.

Also, while its true one could just wire up all of these fixed devices, these wiring costs would massively drive up the cost of implementation. Trenching city streets to lay new cables costs a lot of money, just ask Google.