Chicago's O'Hare International was the nation's second busiest airport in 2012, with more than 310,000 flights.
O'Hare itself was developed to ease overcrowded conditions at Midway, which for years was the world's busiest airport.
The airport's on-time departure performance improved in 2012, rising to 76.6% of all departures from 74.4% in 2011.
