The suitability of an airport site considering an isotropic plain can be viewed as a balance between two opposing forces:
- Benefits. The closer an airport is to the city center, the more benefits are derived from shorter commuting times from the airport to centers of activity. The airport can conveniently service a metropolitan area and maximize the market potential of its customer base. The commuting radius represents a tolerable commuting distance/time from the city center (CBD), which is in the range of 1 hour and similar to the average commuting time. Beyond that threshold, an airport does not serve its metropolitan area well as an undue amount of time must be spent to reach it. The integration of rail systems with airport development, such as in Hong Kong and Paris, reduces the friction of distance by connecting the airport more efficiently to its urban core and region. However, many airports (such as Narita and New York-JFK) have poor connectivity with their metropolitan areas because of congestion and the lack of alternatives to road access.
- Externalities. Locations closer to the city center have more incurred externalities. The opportunity cost for the land devoted to the airport, the number of people adversely affected by noise, and incompatibilities with local land use increase. Externalities have been a strong factor pushing airports away in recent developments, such as Denver and Hong Kong. Under such circumstances, an airport site should be as far as possible from the city center. In the case of Hong Kong, approximately 380,000 people lived within the 65 dB noise contour of the old Kai Tak airport; but no one lived within the 65 dB contour of the new airport when it opened in 1997.
- Suitability. Benefits and externalities functions tend to be inversely proportional. Consequently, a compromise is sought by choosing a site that is close enough to provide significant benefits and far enough to minimize externalities. A location ring of high suitability is derived from an overlay of the benefits and externalities curves.
The real locational context of an airport is obviously much more complex with additional geographical (availability of flat land) and land use constraints, implying that fewer sites may be suitable.