The worldwide air transportation network is responsible for the mobility of millions of people every day. Almost 700 million passengers fly each year, maintaining the air transportation system ever so close to the brink of failure. For example, US and foreign airlines schedule about 2,700 daily flights in and out of O'Hare alone, more than 10% of the total commercial flights in the continental US, and more than the airport could handle even during a perfect "blue-sky" day. Low clouds, for example, can lower landing rates at O'Hare from 100 an hour to just 72 an hour, resulting in delays and flight cancellations across the country. The failures and inefficiencies of the air transportation system have large economic costs; flight delays cost European countries 150 to 200 billion Euro in 1999 alone.
These staggering numbers prompt several questions: What has led the system this close to the brink of failure? Why haven't planners designed a better system? In order to answer these questions, it is crucial to characterize the structure of the world-wide air transportation network and the mechanisms responsible for its evolution. This problem is, however, far from simple. The structure of the air transportation network is mostly determined by the concurrent actions of airline companies – both private and national – that try, in principle, to maximize their immediate profit. However, the structure of the network is also the outcome of numerous historical "accidents" arising from geographical, political, and economic factors.