Vinnie and hayman have given some good design-for-effect answers. I'll give a more literal, representational one:
It kind of depends on how bright the lights were. I realize that's probably not something you're going to be able to know, which means it would really come down to what plays best.
Dim lights would produce an effect similar to E1.942, Illuminating their own Location.
Brighter lights tend to make it easier to see the faces of the nearby buildings. This might produce an effect similar to an E1.94 Illuminated Zone from each (paved) road hex (as if from a one-level Blaze at ground level).
But figuring these zones out is likely to really slow the game, or greatly lengthen the setup process. Depending on the boards it might be easier to just declare the whole map Illuminated.
An in-between version would be to SSR that the (paved) road hexes and the ground levels (only) of all adjacent hexes are Illuminated. I'm guessing that would not slow things much.
In all cases, it would probably be too much to try to model the gradual "blacking out", unless you want that to be the major feature of the scenario (in which case, some version of Clearance and TI would be called for). Otherwise, either have the effects end scenario-wide on turn x, or have them end based on a dr/DR, or have them stay on the whole game, depending on what feels best.
John