O'Hare International Airport opened to commercial air traffic in 1955 and modernized and expanded in 1959. It was developed on an old airfield in a quiet community far northwest of Chicago. The city annexed the land and built the Northwest Expressway into the airport in 1960. By 1961, O'Hare was the world's busiest airport, and many businesses had sprouted up around the site. By the end of the 1960s, industrial parks, manufacturing plants, office complexes, parking lots, and hotels dominated the surrounding countryside.