It's kind of a burned-out version of the 1950's with a flashy 80's aesthetic.
You've seen Guy Standeven in something because the man was in everything.

Strange though it seems this film could also be futuristic. This movie displays anarchy like it's never been known with policemen just watching a street fight, when it could turn out to be a fight to the death. Although I suppose the old wild west was a bit like that.

I know it's pre-titled "another time another place", but I still wonder what they are suggesting.
It's deliberately and stylishly retro-looking because I believe Walter Hill wanted to make a film about the sort of things he saw and enjoyed, when he was a teenager himself.

Many Walter Hill movies look dystopian e.g The Warriors, and there is never a back story. As for where it could be, yes the out of ground subways remind me of Brooklyn and the Bronx. However, the city reminded me of Detroit. Perhaps because of the all the old cars?
As for the backstory, people seem to focus on wars. However, probably was a combination of war and the economy going into the tank. The director lived through the 70s when the economy was a mess and Vietnam had finally ended. In the 70s most major cities were in shambles and NYC was on the edge of bankruptcy.