L.A. Noire, a recent piece from the Los Angeles Times reveals that Rockstar and Team Bondi have expended considerable effort in recreating Los Angeles as it was in 1947. The development team used 1930s era Works Progress Administration maps and topographical information from the US Geological Society to create a city layout.
Beyond the city itself, the developers also strove to accurately represent the lives within it. For example, aerial film and photography from the period was used to recreate authentic traffic patterns. Many other factors were researched, from clothing to the levels of smog that were present in 1947. Head over to the L.A. Times site for the full story.