Where it wasn't a set or a locked off street, it was a digitally "enhanced" location. Pedestrians were removed, dilapidation was added to buildings, all of the billboards and posters were added on, the shrubs and weeds growing through the street were all cg, as were most of the abandoned cars.
I didn't think the effects were that great. A lot of the opening wide shots just looked like photoshopped versions of the real thing. The dilapidation "looked" added on.
I think Children of Men had much much better effects - very similar, but oh so much more convincing.