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| Fig. 1: Proportion of transportation sector energy use attributed to different transportation modes. [1] (Courtesy of the U.S. DOT) |
Transportation is the largest energy-consuming sector in the United States, accounting for roughly 26.214 exajoules (or 24.847 quadrillion BTU) of energy in 2022. [1] Roughly 70% of this energy was used for passenger transportation, or forms of transportation focused on moving people from place to place.
In recent years, increasing attention has been paid to the energy use of the transportation sector, both in terms of the sectors impact on climate change via greenhouse gas emissions and on the risks that increasing energy and fuel prices present to American consumers.
As shown in Fig. 1, cars are the dominant consumer of energy among forms of passenger transportation, making up 60.3% of the transportation sectors energy use. By contrast, air travel is a relatively minor energy consumer compared to cars, accounting for 7.4% of the energy use mix. Public transit, a grouping that encompasses all other forms of publicly-owned passenger transit, accounts for just 0.5% of the sectors energy use. In absolute terms, the energy used by public transit is negligible; the important question is if transit is more efficient on a per-rider basis and how much energy can be saved by displacing other forms of transportation.
While cars and light trucks make up the greatest proportion of overall emissions, we must consider that they also transport far more people than other forms of transportation. To better understand the relative energy use of various passenger transportation modes, we can compare the energy that they use to travel one passenger kilometer (defined as the number of kilometers a given vehicle travels multiplied by the number of passengers moved). These differences are summarized in Table 1.
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| Table 1: Energy use per passenger-mile and passenger-kilometer for major U.S. passenger transportation modes in 2019. [2] |
Rail forms of transportation are the most efficient, using less than 1 megajoule per passenger kilometer. Airplanes are also relatively efficient, using just under 1.5 megajoules per passenger kilometer. Cars and light trucks are relatively less efficient, and transit buses are the least efficient, using over five times as much energy per passenger kilometer as transit rail.
This also indicates that, in practice, air travel is relatively efficient compared to car travel and that transit buses are relatively inefficient compared to most other major forms of passenger transportation.The key challenge behind transit buses' relative inefficiency (and the reason behind air travel's relative efficiency) is the levels of occupancy between the transit modes. Air travel is generally run by private companies which ensure high levels of ridership relative to seats available. By contrast, transit buses are generally run as a public service regardless of the proportion of seats utilized.
For example, FAA data indicates that domestic flights in the United States in 2022 had around 83% of possible passenger kilometers utilized. [1] By comparison, transit buses have an average of 7.5 passengers vs. an average of 49 seats per bus, indicating that around 15% of possible passenger kilometers are utilized. [3]
However, air travel's efficiency depends on flight length to a greater extent than other modes. Short-haul flights are more energy-intensive than long-haul flights because the energy required to reach cruising altitude is spread over fewer kilometers. The values here represent a weighted average of all passenger aviation activity, which naturally places a greater emphasis on long-haul flights. Regional aircraft tend to show higher per-passenger energy use than larger commercial jets. [4] Studies reporting emissions per passenger-distance show the same pattern across aircraft types - and also indicate that low-occupancy car travel often exceeds typical commercial aviation (both short-haul and long-haul flights) in per-passenger fuel use. [5] This confirms the same high-level trend as reported by Department of Transportation data.
Greater ridership of public transit has the potential to decrease the total energy usage of the transportation system, since rail forms of transit are relatively energy efficient in terms of energy used per passenger kilometer. Transit buses are relatively inefficient on a per-kilometer basis on account of their low utilization. Whether transit buses are efficient depends largely on the percentage of seats occupied and the number of alternative modes of transit (in particular, cars) that are displaced.
The average public transit bus consumes 22.868 MJ of energy to move one kilometer and can transport up to 49 people. By contrast, an average car uses 2.814 MJ of energy to move one kilometer and, on average, actually transports 1.5 people.
From these numbers, we can calculate the net change in energy use per kilometer as a function of the number of car users the bus displaces.
With one passenger displaced, the net energy use per kilometer of a bus is 20.992 MJ/km. At full occupancy (49 riders displaced), the net energy savings are 69.062 MJ/km.
Cars are relatively more efficient than buses if the number of people transported is 12 or less, while buses are relatively efficient if the number of people transported is 13 or more. At present, the average number of passengers on a bus is 7.5, making buses relatively inefficient to cars at the present time. [2]
This calculation indicates that forms of public passenger transit (public buses, transit rail, and intercity rail) have the potential to be relatively efficient compared to cars, light trucks, and air travel. The primary challenge in realizing these potential reductions in energy use is overcoming consumers current preferences for cars.
Academic sources link the effectiveness of public transit systems at displacing car use to larger-scale urban planning decisions, especially the prevalence of urban sprawl. [6] In dense corridors, transit vehicles often serve more passengers and operate more efficiently on a per-passenger-kilometer basis. In lower-density areas, origins and destinations are spread out, leading transit services to focus on providing basic mobility services instead of expanding ridership. [7] In these environments, bus services are far less likely to hit the ridership threshold where they are more efficient than cars. As a result, national averages reflect a mix of higher and lower-density environments.
Other proposed alternatives include smaller-scale initiatives to increase consumers likelihood of using public transit. These range from minor behavioral interventions such as increasing car registration fees to larger-scale initiatives to increase the availability of transit infrastructure such as Park-and- Ride or dedicated bus lanes. [8,9]
In order to properly assess the energy use of transportation modes, we need to compare those modes on a relative basis. While transit buses and cars are relatively inefficient compared to air travel in practice, a significant proportion of these inefficiencies are due to the low relative occupancy rates of these vehicles. Additionally, rail (and especially transit rail) is highly efficient compared to all these other modes.
The total energy savings of public transit are heavily dependent on ridership levels, and any policy intending to use public transit as a lever to reduce energy use or emissions must make sure that the system is displacing cars at a high enough level to justify the higher intrinsic energy use of the vehicles.
© David Duncan. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Transportation Statistics Annual Report," U.S. Department of Transportation, December 2024.
[2] S. C. Davis and R.G. Boundy, "Transportation Energy Data Book, Edition 40," Oak Ridge National Laboratory, ORNL/TM-2022/2376, February 2022.
[3] "2024 National Transit Summaries and Trends," U.S. Department of Transportation, 2024.
[4] R. Babikian, S.P. Lukachko, and I.A. Waitz, Journal of Air Transport Management 8, 389 (2002).
[5] L. Chapman, "Transport and Climate Change: a Review," J. Transp. Geogr. 15, 354 (2007).
[6] S. Lyu, Y. Huang, and T. Sun, "Urban Sprawl, Public Transportation Efficiency and Carbon Emissions," J. Clean. Prod. 489, 144652 (2025).
[7] N. Tabassum et al., "Ways of Increasing Transit Ridership - Lessons Learned From Successful Transit Agencies," Case Stud. Transp. Policy 19, 101362 (2025).
[8] Z. Zarabi et al., "Enhancing Public Transport Use: The Influence of Soft Pull Interventions," Transp. Policy 153, 190 (2024).
[9] K. E. Stieffenhofer, M. Barton, and V. Gayah, "Assessing Park-and-Ride Efficiency and User Reactions to Parking Management Strategies," J. Public Transp. 19, 75 (2016).