The first bar is the headline estimate: traffic thins as distance grows. The other bars are reference models with known weaknesses, shown for comparison and not as independent confirmation.
What this shows: The vertical axis shows Distance elasticity (log-log).
Conditions and method
Status: confirmed.
The bars are in a fixed order and are not ranked by size.
The Heckman bar uses the same network controls as the augmented model, which are outcomes of demand, so read it as conditional on them. It is also unidentified: the variable its correction relies on (whether the two airports share a state) fails its relevance test (p=0.48), and the interval of its selection term (the Mills ratio) crosses zero. It is a directional robustness check only.