72 Transportation Research Record: Journal of the Transportation Research Board, No. 2449, Transportation Research Board of the National Academies, Washington, D.C., 2014, pp. 72–78. DOI: 10.3141/2449-08 Airlines reduced available domestic capacity at airports across the United States from 2007 to 2012 in response to a global economic down- turn and high and volatile fuel prices. More recently, despite an eco- nomic recovery and more stable fuel prices, airlines have continued to keep capacity low relative to historical levels in a strategy that has been referred to as “capacity discipline.” The effects of these shifts in capacity on airport connectivity to the global air transportation network remain unclear. This paper introduces an intuitive index to compute airport con- nectivity as a function of both the quantity and quality of scheduled non- stop and connecting service. Connectivity scores were computed for 462 U.S. airports; medium-hub and small-hub airports were found to have lost more connectivity on average than large-hub airports from 2007 to 2012. In multiairport regions, losses in connectivity at secondary and tertiary airports outpaced losses at primary airports. With lower lev- els of connectivity, smaller airports in these regions will need to employ creative strategies to prevent passengers from leaking to primary hubs. Whether these airports can recover from connectivity consolidation will also depend on how long the capacity discipline equilibrium remains in effect among U.S. airlines. As the pace of globalization has increased in recent years, commercial air service that provides connections to the global air transportation network has become increasingly important for economic, social, and demographic reasons. While air connectivity is important for communities of all sizes, research has suggested that small commu- nities can obtain significant economic benefits from well-connected commercial air service (1–3). The U.S. air transportation system has undergone a series of changes in response to the financial crisis of 2007 to 2009 and asso- ciated high and volatile fuel prices. Despite an economic recovery and more stable fuel prices, airlines have continued to keep capacity low relative to historical levels in a strategy that has been referred to as “capacity discipline” (4). More than 14.3% of yearly scheduled domestic flights were cut from the U.S. air transportation network from 2007 to 2012, mostly resulting from actions of the network carriers (4). Smaller airports were disproportionally affected by the cuts in service, losing 21.3% of their scheduled domestic flights from 2007 to 2012 as compared with an 8.8% decline at the 29 largest U.S. airports (4). However, simply examining gains or losses in flight volumes does not provide a complete picture of the strength of commercial air service at an airport. For instance, many smaller airports lost ser- vice from network carriers from 2007 to 2012 but saw new service from ultralow-cost carriers (ULCCs) such as Allegiant Air or Spirit Airlines. These ULCCs typically provide infrequent departures to vacation destinations and offer limited connecting service to other U.S. airports or the global air transportation network. The small airports that lost network carrier service only to receive replacement service from ULCCs may not have seen significant decreases in flight volumes, but their connectivity was likely adversely affected. It is also valuable to analyze which airports have seen increases or decreases in connectivity on a regional level over a given period. As airlines have consolidated service at larger airports in multiairport regions, so too has connectivity been concentrated at large hubs, often leaving smaller airports at a disadvantage. Although many research- ers have analyzed connectivity and accessibility to air service, there is currently no industry-standard metric to assess an airport’s con- nection to the global air transportation system. This absence creates challenges for airport managers and policy makers in interpreting the effects of gains or losses in service on an airport’s connectivity. This paper introduces a new, easy-to-compute index metric that is used to assess such changes in connectivity at U.S. airports. The index computes airport connectivity as a function of the frequency of available scheduled flights and the quantity of nonstop and con- necting destinations. Unlike many connectivity models, the index also considers the quality of destinations served, such that an addi- tional flight to a large city or a major connecting hub is more valu- able than an additional flight to a smaller community with limited connecting options. Connectivity scores for the years 2007 to 2012 were computed for 462 U.S. airports. Medium-hub and small-hub airports suffered the greatest losses in connectivity from 2007 to 2012. Connectivity index scores at medium-hub airports fell by 15.6% on average between those years, compared with an 11.0% decline in connectivity at small- hub airports and a 3.9% decline at large-hub airports. The decline in connectivity can be attributed to airlines cutting capacity and desti- nations as a result of challenging macroeconomic events, the move- ment of Southwest Airlines toward larger primary airports, industry consolidation, and more restrictive capacity management strategies across the industry. It should be noted, however, that percentage changes in connec- tivity at most airports were less than percentage changes in flights or available seats. This fact suggests that some of the service cuts as a result of capacity discipline strategies did not directly harm con- nectivity, but instead removed redundant flying to secondary hubs. At many small airports, removing a flight to a secondary hub would not result in a substantial loss in connectivity as long as flights to other, larger connecting hubs remain. Capacity Discipline and the Consolidation of Airport Connectivity in the United States Michael D. Wittman and William S. Swelbar M. D. Wittman, Building 35-217, and W. S. Swelbar, Building 33-318, International Center for Air Transportation, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139. Corresponding author: M. D. Wittman, wittman@mit.edu.