Transportation in the United States is the largest sector contributor to greenhouse gas emissions (GHG) (EPA, 2020). The proportion of passenger miles for long-distance, intercity travel is estimated to be 30% of the total for air, on-road passenger vehicle, and other modes. Methods for estimating and mitigating these emissions have received limited focus and there is a need to evaluate if a reduction in long-distance travel emissions is feasible through behavior change including modal shift, vehicle occupancy, or destination choice. The focus of this research is the design and implementation of a carbon calculator that estimates the CO2 emissions of both passenger vehicles and aviation options for surveyed real-world trips. This model facilitates assessment of when it is more efficient to drive or fly. The Vermont Intercity Carbon Calculator (VICC) increases the accuracy of trip mode-based emissions for long-distance trips by accounting for airport operations, ground side equipment, airport access/egress, and routing which embodies different numbers of take-offs, landings, and taxiing operations.
The VICC was applied to 2,112 one-way trips in the continental US over 250-miles from the Longitudinal Survey of Overnight Travel (LSOT) reported by 697 people between February 2013 to February 2014. The data included origins as home locations and destinations as the farthest stop of a trip. The 40 major airports included in the model represented large and medium hubs and also small hubs and non-hubs that were a longer distance from main hubs. These airports were chosen to allow for a realistic approach for determining average air access between origins and destinations for trip decision-making. The results show that the additional variables introduced to the VICC carbon calculator for access and egress travel and airport operations account for about half of the total emissions of an individual trip using air. This demonstrates that the current carbon calculators are missing a significant amount of emissions from these sources and non-primary mode travel and air routing should be accounted for. While the model and these emissions estimates for air trips do have uncertainty due to emission factors and other assumptions, a large portion of the variability evident in the model output is attributable to real variation in the aviation system and services: routing, plane type, and airplane passenger load.
This realistic variability in trip-based emissions results in a range of distances for which it is more efficient for a traveler to drive or fly for their long-distance trip. When traveling less than approximately 190 miles, passenger vehicle travel can be confidently assessed as more efficient compared to aviation as the primary travel mode. For long-distance trips greater than approximately 916 miles, aviation can be confidently assessed as more emissions efficient for a given trip. Countermeasures for the overall system could include increasing the efficiency of aviation by decreasing the number of flights offered to consumers with low passenger loads and halting short-haul flights even those which are part of longer trips.
As participants reported which mode was used for their travel it is possible to delineate who made the more efficient mode choice for their particular origin and destination. Of all 2112 trips, 80% were made by the mode modeled as more efficient and these tended to be air. This points to an additional critical factor in system optimization: emissions per trip embody a vehicle occupancy and a total trip distance. While policy efforts can be made to encourage more emission-efficient mode choices per unit distance or trip, destination choices may be more important than individual mode choice decisions for efficient travel.