I'm not sure I'd get too hung up with the physics and what would actually be visible from a given height (not much, really). Rather, the goal is to replicate the pretty long-exposure Blue Marble images.

I did something similar relatively recently. You don't want to try to make the glow with your brush. Make tiny, hard dots, layer them up, and then apply the glow at the end. The difficult part is in getting the plausible spider-webby road networks and distributing the dots over them in a way that looks right.