Environmental protection initiatives in the air travel sector encourage passengers to consider their personal emissions footprint in selecting travel itineraries. These initiatives mainly rely on average emission factors (AEFs) whereby air passengers are anticipated to choose itineraries with lower AEFs. However, these AEFs cannot represent the marginal impact of changing demand on emissions, and thus, misinform travelers and policy makers. In order to capture this marginal impact, we estimate marginal emission factors (MEFs) on domestic U.S. routes that originate from different airports within driving distance and arrive at common final destinations. We do so by applying a Breguet Range Equation-based emissions model on publicly available air operations data for the selected routes. Compared to routes originating from large hubs, MEFs on routes originating from small and medium hubs are 63%-105% and 36%-65% higher, respectively. However, AEFs on routes starting from small and medium hubs are only 27%-50% and 18%-40% higher, respectively, compared to those starting from large hubs. These findings indicate that AEFs significantly underestimate the emissions savings of choosing itineraries that originate from large hubs over those that originate from smaller airports. Therefore, it is critical to incorporate MEFs in environmental initiatives in the air travel sector.
