11.8 Efficiency Per Passenger Mile

The chart below compares different types of transportation using the same unit: gasoline gallon equivalents per passenger mile. This makes it possible to compare transportation modes on a common basis, even when they use different energy sources such as gasoline, diesel, or electricity.

A passenger mile means moving one passenger one mile. Looking at efficiency this way helps us compare how much energy is used to move people by car, bus, train, or airplane.

The Average Per-Passenger Fuel Economy by Travel Mode graphic with descriptive text is available from the U.S. Department of Energy's Office of Critical Minerals and Energy Innovation site.

Looking at the chart above, transit rail appears to be one of the most efficient ways to move passengers. Rail systems are often efficient because trains can carry many people at once, experience relatively low rolling resistance, and in many cases use electricity.

Air travel may also appear more efficient than some people expect on a per passenger mile basis. Although airplanes use a large amount of energy overall, many flights carry a high number of passengers. When that energy use is divided among many riders, the energy use per passenger mile can be lower than expected.

You may be surprised that transit buses can have relatively low efficiency on a per passenger mile basis. One reason is that buses often run fixed routes and schedules whether they are full or nearly empty. If only a small number of people are riding, the energy used by the bus is divided among fewer passengers, which lowers efficiency per passenger mile.

Demand response vehicles, such as taxis and ride-share vehicles like Uber or Lyft, can also be less efficient. These vehicles may need to travel extra distance to pick up passengers, which increases the amount of energy used for each trip.

Key Takeaways

Transportation efficiency is not just about the vehicle itself, but it also depends on how many passengers are riding, how far the vehicle travels, whether the vehicle runs full or mostly empty and what type of fuel it uses.