Source: Airports Council International.
The North American passenger and freight air transport system is designed around the hub-and-spoke network structure. There is a significant divergence between the activity level of passenger and freight airports. The activity level of passenger airports is related to the distribution of the population with airports having a higher activity level either because they are the hubs of major airlines or are important resort areas (e.g. Las Vegas, Orlando). For instance, Delta mainly operates hubs at Atlanta (ATL), Detroit (DTW) and Minneapolis (MSP), while United mainly operates hubs at Denver (DEN), Houston (IAH), Chicago (ORD), Los Angeles (LAX) and Newark (EWR). American Airlines mostly operates hubs at Dallas-Fort Worth (DFW), Chicago (ORD) and Miami (MIA).
The activity level of freight airports has two dimensions. The first concerns the major commercial gateways of North America (e.g. New York, Miami, Los Angeles, Chicago, Toronto), which are at the same time major consumption markets. The second concerns the hub-and-spoke strategies of air freight integrators. The Midwest is the demographic and economic centroid of the United States, many air freight integrators have located their hubs at airports such as Memphis (Federal Express) and Louisville (UPS) that generate substantial cargo flows, but little passenger traffic.