How does the air traffic network evolve over time? Is there any pattern to how the air traffic network evolves with respect to the size of the markets, type of competition, geospatial features, and structural changes following the events of September 11, 2001 (9/11)? Are the changes transitory or permanent in nature? Can we lay out the trajectory possibilities of the network and determine factors influencing them? The National Airspace System (NAS) in the United States US) is structured primarily around a web of air transportation markets linked to each other through a network of 465 commercial airports located in and around 363 metropolitan statistical areas (MSAs). The total number of origin-destination (OD a small number of markets account for the largest number of passengers and, hence, air traffic flows. For example, there were approximately 105 markets (0.3% of the total) which had 1,000 or more passengers a day (i.e., thick markets), but these accounted for almost 17% of the total passengers. On the other hand, there were almost 28,000 markets (78% of the total) with 10 or fewer passengers a day that accounted for only 6% of total passengers in 2003. These O&D market pairs have been served, generally speaking, by 52,000 − 56,000 flight segments (i.e., routings passengers took to travel the markets) depending upon the extent and intensity of network. In recent years, however, the network segments have increased sizably to an average of 67,000 − 72,000 segments leading to increased fragmentation. 1 Understanding the evolutionary nature of the airline network is extremely important. Investment decisions with consequences stretching far into the future are being made to serve the airline network. A proper understanding of the dynamic nature of the airline network, therefore, is essential to minimize costly mistakes. We used a multinomial logit model to analyze and determine itinerary choices in the US scheduled airline industry. Using 10% ticket sample data for the second quarter of 2003, we find that passengers, weighted average fare, average distance and types of air carriers empirically determine the itinerary choices. This simple model captures lower-order itinerary choices (i.e., those with less than three stops) fairly well for the sample of almost 360,000 itineraries.
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