Transportation systems move people. They also move freight—stuff—including raw materials and manufactured goods. People move themselves around mostly by means of cars, buses, planes, trains, and bicycles, while we mostly move our stuff with trucks, trains, ships, and planes.
By weight, we move far more stuff than people: the combined weight of all the people in the U.S. is about 24 million tons, while in 2012 we moved nearly 20 billion tons of stuff (roughly 800 times as much). But while people weigh so (relatively) little, over 70% of the total energy consumed by transportation in the U.S. that year went to moving us, not our things, around.
According to the International Energy Agency, the amount of useful energy that human societies consumed in 2013 totaled 9.3 billion metric tons of oil equivalent (108,159 terawatt-hours). Of that total, 28 percent was used for transportation.
One resource dominates operational energy in the transportation sector: oil, from which we make different types of liquid fuels for our various vehicles. Gasoline fuels the vast majority of passenger cars (of which there are roughly a billion worldwide). Diesel powers trucks, buses, about half of the world’s trains, and some automobiles. Bunker oil fuels large ships, while most commercial aircraft burn aviation-grade kerosene.
About half of the world’s trains are electric (the proportion is much smaller in the U.S.), and electricity also powers subways, streetcars, and a growing number of electric buses, cars, and bicycles. But even so, nearly 93% of transportation is fueled by oil.
While most of the energy consumed by transportation is in the operation of vehicles, a great deal of energy—the vast majority of it from fossil fuels—is embodied in the manufacturing and maintenance of vehicles, roads, rails, parking structures, and airports. (This embodied energy is not accounted for in the transportation total above.)
Each typical automobile represents about 48 megawatt-hours (MWh) of embodied energy, or the equivalent of 29.5 barrels of oil. That means manufacturing the nearly 90 million new vehicles produced globally in 2014 required more energy than the amount of total renewable electricity produced in the world that year: 4,300 terawatt-hours (TWh) vs. 3,685 TWh.
Roads also embody large amounts of energy: the energy used in constructing one lane of road one kilometer in length is the equivalent of burning 23,000 gallons of conventional gasoline. Thus the global network of 65 million kilometers of paved roads represents about 1.5 trillion gallons of gasoline in embodied energy.
All this is to say that, even if we were able to substitute vehicles that run on petroleum—planes, trains, automobiles, and ships—with ones that ran on renewable electricity, we’d still have the challenge of substituting all the fossil fuels that go into their manufacture, maintenance, and disposal, along with the infrastructure (the roads, bridges, parking, etc.) that supports them.
Example: A Hybrid Electric Car
On the whole, passenger vehicles are getting more efficient (though, put in perspective, driving a car remains a woefully energy inefficient mode of transport)—with the average fuel efficiency of a new car sold in the U.S. now at a little over 25 mpg. In comparison, the most popular “green” car on the market—the Toyota Prius—gets double that. It’s a big improvement. But what about the energy needed for everything other than driving the car?
Manufacturing a Prius requires sourcing and transporting raw materials, including steel, glass, copper, cast aluminum, lithium, synthetic rubber, plastics, magnesium, and platinum. Electricity was used in some of these sourcing operations, but the overwhelming majority of the energy used was in the form of natural gas, oil, and coal.
The maintenance of the vehicle requires replacement parts, lubricating oil, tires, a repair facility (running largely on electricity), and transport of parts, oil, and tires (more oil). The tires are made of a synthetic rubber derived from oil, about one barrel of which goes into manufacturing each tire.
Although the Prius has an electric motor on board, it cannot be plugged in so as to charge its battery with electricity from an external source—instead, its battery charges from the kinetic energy captured while the vehicle is braking. Its electric function serves merely to increase its fuel efficiency. The car’s operational energy therefore comes entirely from gasoline.
We would not be able to operate the car if it were not for an extensive system of roads and bridges. National, state, and local governments spend enormous sums annually to build and repair these roads, using asphalt, steel, concrete, lumber, and aggregate; the energy fueling this road-building and repair activity is overwhelmingly from oil.
Finally, when the car reaches the end of its useful lifetime, it will require disposal at a facility dedicated to this purpose. Energy (in the forms of electricity and oil) will be expended in transporting the car, demolishing it, and recycling its components.