Absolutely no idea. It's never specified (either in the game or in the player's guide, which gives a few more clues about the setting). I've always seen it as being like the setting to Mad Max or Marisol, though...the "five minutes into the future" idea. You know, it has elements from our present, but also has some random futuristic elements (like the hover-cars) thrown in here and there to trip you up.
I think you could safely say that it could be in an alternate reality mid-90's (since that's when the game came out)--you know, a reality where they had an apocalyptic war somewhere along the line, and then decided to invent hover-cars.