The nerd returns....
My experiment with extracting a city light mask from USGS land cover data worked OK, but the resolution was a little limited. Started to download the transport layer from the USGS to have a bit of a play. While roads alone don't make up city lights, I worked on the basis that more roads = more lights. Using a combination of layers, blurring images, masking layers and resampling (all pre TG) I came up with this quick test.
This isn't too bad, but some extra masking is needed to dim roads based on the road density. I have some ideas on how to achieve that.... and that will also let me leave more defined roads in built up areas as well. Running a render with some clouds to test some GI settings. Don't hold your breath though... we all know it's going to take a while to render. (detail 1, aa7, GI 2,4, envirolight strength on atmosphere 2.5)
Don't ask me which city it is as I don't know. It's SE of Seattle and on a big'ish river.