Several friends asked me why ATL is the world busiest airport in terms of passengers in that Atlanta is neither as big as New York or Los Angeles nor is more important than Chicago in terms of economy. The only reason I can think about is that ATL is Delta Airlines (the world No. 3)'s biggest hub. However, this might not be convincing because ORD is UA's biggest hub and AA's second biggest hub and more international airlines serves ORD than ATL. One of my colleagues told me that another reason is that ATL has the most efficient airport traffic control system supported by Georgia Institute of Technology. Is this true? Or are there any other reasons for ATL's performance?

Thanks!