Tyler Cowen tries to get at the effect an individual passenger has on airline route decisions and thus the environment.

A simple model of route expansion is that higher demand increases the number of total flights by some probability. For the system as a whole, the decisive flying unit has to come somewhere and there is no reason why, on average, it can’t be you. In other words, at least in stochastic terms you can’t escape the blame.

A second simple model of route expansion is that gates and other airport facilities are scarce and underpriced relative to demand. When demand goes up, supply is not very elastic and mostly they raise price rather than increasing output. Those who feel very guilty should prefer this second model.

Though still imperfect, I’d contend we can do much better in pinpointing the likelihood that traveling on any individual flight makes some contribution at the margin to an airline’s decision-making calculus. Your effect on airline expansion will on average vary proportionately to the fare bucket you’re buying your ticket out of. You can approximate your marginal impact on a flight segment by flight segment basis.

If you’re pulling inventory out of a low fare bucket, the strong expectation is that there’s little effect at the margin on your buying the ticket because the airline expects to operate a flight that doesn’t come close to filling up.

If you’re pulling inventory out of a high fare bucket, for coach fares at the extreme end if you’re traveling on a Y fare, you can pretty much expect that the flight will be close to sold out and that the airline is willing to risk displacing another passenger in the short term in exchange for your higher fare… or at least that the ticket cost is high enough to potentially influence behavior on the part of the airline (you’re part of a small subgroup of passengers paying the highest fares that airlines crave and will make their decisions based on the relative mix of such passengers rather than on passengers as a whole… whereas if you’re in a low fare bucket they’re just scooping up some incremental revenue for a flight they’re planning to operate for other reasons).

Reality is even a little bit more complicated than that. Cargo has to come into play, too. Regardless of what you pay and what fare class you’re booking in, your travel on United between San Francisco and Nagoya, Japan is going to have almost no effect whatsoever on United’s decision-making. They’ve got a very large contract with Toyota and they fill up their 747 with cargo and the flight goes out with very low load factors yet is still profitable for them to operate. That’s why the single easiest flight in the entire United system on which to score an international premium class award ticket on is San Francisco- Nagoya….

Meanwhile, speaking of award tickets if you’re traveling on an award at the saver level that’s the extreme limit of the belief on the part of the airline that they would (a) otherwise operate the flight and (b) that your seat would go unsold. Hence in the domestic case they’re billing the frequent flyer program only the true marginal cost of your travel rather than some average cost figure.

If you are traveling on saver-level award tickets you can be quite confident your environmental impact is quite small, limited for the most part to the extra cost of fuel driven by having your weight onboard the aircraft (and quite possibly outweighed by the fuel you’d be consuming were you not flying that day).

And if you’re paying for your ticket, and you want to be confident that your travel decision won’t effect route planning decisions, buy your tickets (a) close to the date of departure, the closer the better so that the airline’s guess about your effect will be far less speculative and (b) when the lowest fare buckets are showing wide open availability.  Check the fare basis or ask your travel agent. 

For example:

• Delta, look for an L, U, or T fare.

• United, perhaps an S, T, K, or L fare.

• American, ideally an S or N fare.