The Hub Power Index
Six airports account for a disproportionate share of U.S. air traffic. But scale alone doesn't capture the story — carrier concentration at some hubs has reached levels that rival monopoly thresholds, and it's still rising.
The Question
Which U.S. airports are structurally dominant, how concentrated is carrier control at each hub, and how has that concentration changed since 2020?
Hub dominance matters for three reasons. First, it predicts fare environment — highly concentrated hubs tend to have higher average fares than competitive markets. Second, it shapes route availability, since a single dominant carrier controls what gets served. Third, concentration trends tell you whether competition is improving or eroding over time.
Precise Snapshots with the T-100 Traffic API
The traffic/segments/breakdown endpoint draws on the T-100 dataset — a 100% census of all scheduled carrier operations filed monthly. Unlike ticket data, this is operational data: passengers, departures, and ASMs at the flight-segment level.
For the 2020–2025 concentration trend, we use a targeted annual snapshot strategy: one precise query per June, per year. Each call is fast and focused, and together they build exactly the multi-year trend we need.
- Snapshot:
- year=2025&month=10 (single period)
- region:
- D (domestic only)
- service_class:
- F (scheduled passenger/cargo service)
- Metrics:
- passengers, departures_performed, available_seat_miles
For the concentration trend, we repeated this query with by=carrier&origin=ATL for each hub, once per year (month=6 for June), for 2020–2025. Each query is a single-period result. Six queries × six hubs = 36 API calls, each taking <500ms.
Airport Scale: Who Controls the Network?
The treemap below shows October 2025 domestic passenger volume for the top 50 airports. Rectangle size is proportional to passengers; the distribution follows a classic power law — the top five airports carry 21.29% of all domestic traffic in this snapshot.
U.S. Airport Traffic Share — Treemap
Top 50 airports by domestic scheduled passengers · T-100 · October 2025
/traffic/segments/breakdown?by=origin®ion=D&service_class=FATL, DEN, ORD, DFW, and LAS are in the top tier in this October 2025 snapshot. Just behind them are LAX, PHX, CLT, and MCO (see the LCC analysis).
Carrier Concentration: The HHI Picture
Traffic scale tells you who’s big. Carrier concentration tells you who’s powerful. We compute the Herfindahl-Hirschman Index (HHI) for each hub using the formula:
HHI = Σ (carrier_share / 100)² × 10,000
DOJ thresholds: HHI > 2,500 = highly concentrated · HHI 1,500–2,500 = moderately concentrated · HHI < 1,500 = competitive.
- Six hubs queried:
- ATL (DL dominant), ORD (UA dominant), DFW (AA dominant), DEN (UA dominant), IAH (UA dominant), CLT (AA dominant)
- Annual snapshots:
- June 2020, 2021, 2022, 2023, 2024, 2025
Repeated single-period queries avoid the 24-month range limit. June is chosen for consistency: summer peak months have the most complete carrier representation. The service_class=F filter keeps only scheduled passenger operations.
Carrier Concentration at Major Hubs — HHI Over Time
Annual June snapshot · T-100 · 2020 – 2025 · Herfindahl-Hirschman Index (HHI) measures carrier concentration
/traffic/segments/breakdown?by=carrier&origin=…&month=6 per year 2020–2025Select a hub using the buttons to see carrier share by year plus the HHI trend. The dashed yellow line is HHI on the right axis — watch how ATL’s HHI reaches 5,944 by 2025 as Delta’s post-pandemic dominance increased. For context, a monopoly would score 10,000.
ATL is the most striking case: Delta’s June 2025 share reached 76.6%, producing an HHI of 5,944. DEN is the least concentrated of the six hubs shown (HHI 2,727), while CLT remains highly concentrated with American at 69.0% share and HHI 4,959.
Hub Connectivity: Who Flies Where
Hub concentration sets the competitive context. Destination diversity tells you what that concentration buys travelers. The chart below shows the top 15 destination markets for each hub — select a hub to see its connectivity profile.
- Snapshot:
- Single period: year=2025&month=10
- limit:
- 50 (single-period queries allow up to limit=100)
Single-period queries allow limit=100 — much higher than the 25–50 limit for multi-period range queries. This gives a more complete picture of the destination network without missing tail markets.
Hub Connectivity — Top Destination Routes by Passenger Volume
T-100 · October 2025 · top 15 destinations per hub
/traffic/segments/breakdown?by=destination&origin=…&limit=50ATL serves the widest range of destinations — its top markets are mostly Southeast and East Coast cities, reflecting its role as the dominant regional hub. DEN shows an interesting split: coastal markets (LAX, SFO) and Mountain West leisure destinations (PHX, LAS) alongside major hub connections. CLT is heavily oriented toward East Coast spoke cities, consistent with American’s regional hub strategy.