Most US routes are one-airline monopolies (about three in five today), and the mix has shifted only slowly over three decades.
What this shows: The chart covers 1990 to 2024. It plots Monopoly, Duopoly and Competitive (3+) on a vertical axis of Share of routes (%). Latest values: Monopoly 61.1; Duopoly 18.5; Competitive (3+) 20.4. Data run through 2024.