Source: Airports Council International. Note: Several metropolitan areas contain more than one airport.
Measuring the importance of a metropolitan area in the global air transport system using the passenger or freight traffic of its largest airport can be misleading. This is particularly the case for large metropolitan areas with more than one airport, implying a distribution of the traffic. For instance, while Atlanta has the largest passenger airport in terms of traffic, it is London and New York, with 3 major airports each, that account for the largest amount of air traffic handled by their metropolitan areas.
The main reasons that large cities may have more than one airport can be linked to demand for air travel beyond the capacity of the initial airport, requiring the construction of new airports. Many airports that are closer to the central business district were built decades ago and had no additional footprint for expansion. There can also be competing jurisdictions within the metropolitan area seeking to develop and expand their airports.