Energy use in transportation

Transportation, the movement of people and goods, helps shape society and accounts for a large percentage of the world’s use of energy. Transportation methods have evolved from simple applications of animal and wind power to an intensive worldwide use of fossil fuels to power ships, mass rail transit, individual automobiles and trucks, and airplanes. Concerns about resource depletion and environmental effects dictate that all forms of transportation become cleaner and more efficient.

Background

Transportation uses tremendous amounts of energy. Roughly 25 percent of the world’s energy is used for transportation. Among the most industrialized countries, the figure is higher—roughly 30 percent. In 2022, the United States used 27 percent of its total energy production for transportation. It is estimated that 55 percent of the world’s oil is in the transportation sector, and 85 percent of that consumption is by road vehicles. In 2024, researchers estimated that there were more than 1.4 billion vehicles in use across the planet.

The Beginnings

Human muscles are the oldest energy source for transportation. They alone served until the earliest civilizations learned to harness draft animals and invented sails. Draft animals provided the power for great overland migrations, and advances in technology changed history. The development of stirrups, for example, helped invading Goths swing battle-axes from horseback and ultimately smash the Roman Empire. Horseshoes allowed greater range generally and provided greater endurance on hard city streets. Harnesses that allowed draft animals to pull without being choked (along with better plows) increased the food supply in medieval Europe. Draft animals are still important in many areas of the world.

Sailboats, the second major invention, allowed small groups of people to make incredible voyages and settle distant lands. Sail-driven commerce fueled several commercial and industrial revolutions. Ancient Mediterranean fleets brought key materials from as far away as Britain and India. Roman shipping allowed commercial farming in Egypt and Libya to support manufacturing elsewhere around the Mediterranean. Rome fell, but sailboats continued growing in size and sophistication. Later, advances in control technologies such as compasses, jib sails, and accurate clocks (allowing calculation of east-west location) fostered worldwide sail-driven commerce. Still, sail ships tended to move expensive cargoes: spices, silk and other textiles, china, whale oil, tobacco, rum, and enslaved people. Size was limited by mast height and worries of huge becalmed sail ships being run aground by currents.

Water Transport

James Watt’s improvements to the steam engine in 1782 allowed the development of mechanically powered transportation. The first commercially successful use of a steamboat was by Robert Fulton in 1807. Steamboats opened river systems to trade and conquest. By the late nineteenth century, steamships had replaced most sail craft at sea because steamers could be bigger, could move against the tide, and could move without wind. Steady improvements to Fulton’s steamboat included the replacement of paddle wheels with propellers, the use of turbines instead of pistons for expanding steam, coal fuel giving way to oil (oil has greater energy density), steel construction, radar, and global communications.

A number of possibilities, with various degrees of feasibility, exist for improvement of energy use in water transport. These include a greater use of catamarans, hydrofoils, hovercraft, and submarines. A catamaran is a type of boat design that minimizes the energy-expensive bow wave that cuts through the water’s surface by use of two hulls below the waterline. Two thin hulls (creating two relatively small bow waves) breach the surface and support a broad platform. Catamarans can increase speed or decrease energy costs, and the broad platform has many uses; a catamaran design could be used for cruise ships, drilling platforms, and aircraft carriers. A prime disadvantage is a deeper draft than conventional ships, a problem in many ports.

Hydrofoils reduce the bow wave with comparatively small underwater wings that lift the main hull out of the water. Hovercraft have blowers maintaining a cushion of air under the ship. Both hydrofoils and hovercraft are more energy intensive than conventional craft. However, they can be more efficient than aircraft for short and intermediate ferry routes, such as across the English Channel, around the Baltic Sea, and among the Hawaiian Islands.

Submarines avoid bow wakes entirely, and they can avoid all surface conditions. Submarines can also run under the Arctic ice between Europe and Asia, thereby shaving thousands of kilometers from shipping distances. In theory, submarine tankers could be used to transport petroleum and natural gas from Arctic wells year-round. The greatest barrier to the use of submarine transport vessels is the tremendous expense of financing such a radical change in commercial ship construction.

Rail Transport

George Stephenson ran the first commercially successful steam-powered railroad locomotive in 1829. By the late 1800s, a transportation revolution had tied the continents together with steel rails. In the United States, the Midwest could supply the country with grain and meat, while manufactured goods were provided by factories in the East.

At the same time, the British were embracing railroad transport for both shipment of goods and transport of people from place to place. Unlike in the United States and Canada, British rail development, funded by private investors, happened by building short rail stretches, with the network expanding as need arose. In Europe, rail development occurred in most places at the hands of government.

Evolution in railroad equipment has included replacement of steam with cleaner and more-powerful-per-unit-weight diesel locomotives or electric engines, as well as better rails, lighter cars, and sophisticated communications for real-time monitoring of shipments. In the future, fuel cells may replace diesel power because of their greater fuel efficiency and cleaner exhausts.

Particularly in North America, rail passenger service has been hurt by the widespread use of airplane travel. The Europeans and Japanese, on the other hand, have maintained rail passenger service and pioneered high-speed rail travel, making rail travel more popular than airplane travel for trips of 965 kilometers or less. In 1964, Japan introduced the first high-speed train, which traveled at speeds of 209 kilometers per hour. Germany introduced high-speed service in 1965, achieving similar speeds. The introduction of magnetic levitation (maglev) trains, which reduce friction and noise, has increased speeds at which trains travel to more than 480 kilometers per hour. In 2009, German-designed trains began running in Russia, and that country joined Germany, France, Japan, Taiwan, South Korea, China, Italy, Britain, and Spain as countries with high-speed rail networks.

In the United States, the American Recovery and Reinvestment Act of 2009 included $13 million for a high-speed rail program, opening the possibility for an upgraded U.S. rail network in the future. In all cases, fully used passenger rail systems are many times more energy efficient and less polluting than aircraft. For moving freight, both rail and shipping have been made many times more efficient because of containerization and “piggybacking.” A standardized shipping container can be loaded onto a truck at a factory. The truck trailer can be piggybacked onto a railroad flatcar at a railhead. Then the piggyback trailer can be unloaded and driven to a dock. The container can be loaded onto a ship, then perhaps offloaded to another train and another truck before reaching its destination. All those connections are made with minimal expense and handling.

Pipelines

The pipeline, the most efficient method of moving cargo, was put into large-scale use during World War II after German submarines began to attack oil tankers along the Gulf and East coasts. The response was a long-distance pipeline on land. Pipelines are the preferred method for moving large amounts of oil and natural gas. Coal has also been moved in slurries with water, and in theory many raw materials could be moved in pipelines. The greatest barrier to such pipelines is the amount of water required and the fact that the water used must be cleaned or recycled.

Revolution in the Cities

The expansion of urban transit began in the 1800s with draft animals being used to pull streetcars. Soon, traction from centrally powered cable systems (as in San Francisco), and then the use of electricity supplied by overhead cables, or a “third rail,” increased power and speed and eliminated horse manure from city streets. Trolleys (light rail transit) are still the cheapest way to transport people in a city. The more ambitious variant, subways and elevated railways, can move the most people. Both are more efficient than buses and vastly more efficient than automobiles.

In 1884, James Starley’s improved bicycle design provided another means of commuting. The later development of pneumatic tires and gearing provided individual transportation as fast as a horse, with much less maintenance. Consequently, bicycles became a significant means of transportation. Bicycles are widely used in Asian countries and a number of European countries. In developed countries, the comfort and other advantages of automobiles have decreased bicycle use. Continued improvements in bicycles (including lighter materials and aerodynamic fairings) suggest that pedal-driven vehicles could reclaim market share from cars for many short commutes, saving energy and reducing pollution. However, this change is unlikely to happen at a large scale at places and times where fuel prices are low.

A transportation revolution that complemented trolleys, trains, and bicycles involved vertical transportation: elevators. The climbing of stairs to live or work in a tall building becomes steadily less practical after about four stories. Various lifts were developed throughout the centuries, but they all had the weakness that a break in the cable would send the car hurtling to the bottom level. This became a major concern in the nineteenth century when builders wanted to build taller buildings and had steam power available to power elevators. Elisha Graves Otis’s safety device (1852) involves a brake that is locked from engaging by tension from the elevator cable; if the cable slackens, the brake engages and the elevator stops. The safety elevator made skyscrapers practical, allowing the greatest concentration of jobs and people the world has ever seen. By the mid-twentieth century, the New York skyline epitomized wealth, power, and progress. Then the effects of another transportation revolution began spreading society back out in less concentrated patterns.

Automobiles and Trucks

The development of the internal combustion engine made powered vehicles smaller than trains possible and practical. The four-stroke Otto cycle (developed by Nikolaus August Otto in 1862) burned gasoline, and the two-stroke Diesel cycle (Rudolf Diesel, 1892) burned a heavier oil fraction. Trucks and automobiles provide the flexibility and economic advantage of moving small numbers of people and goods to areas away from ports and railheads. This development fueled prosperity and sprawled settlement over great suburban distances. This pattern is typified by low-density metropolitan areas such as Los Angeles, California, and Phoenix, Arizona. Even older cities are ringed with satellite cities growing faster than the core city because surface transportation allows suburbs to have many of the benefits of the city without the costs and problems. Because of widespread use of automobiles and trucks in developed countries, the efficiency of these modes of transport is crucial for overall energy and materials efficiency.

Trucks and automobiles have always been inefficient compared with mass transit, but their convenience has made them so successful that they have come to cause considerable problems, even dangers, for modern society. They depend on oil (much of it from unstable regions such as the Middle East), and they cause air pollution and congestion. Because short hauls of cars and trucks are the dominant energy consumer among transportation methods, and because their use is growing so rapidly, they must be made more efficient and less polluting.

Automobiles in the United States have increased in efficiency. In the 2009 Cash for Clunkers program, funds were allocated as rebates to citizens who traded in their “gas guzzlers” for more fuel-efficient cars. Approximately 700,000 old vehicles with an average rate of 6.7 kilometers per liter were removed from the road and replaced with vehicles with an average mileage of 10.6 kilometers per liter. The improvements come from lighter and more aerodynamic cars, smaller engines, better tuning, and radial tires, which decrease rolling resistance. Further improvements in automobile efficiency and flexibility are being seen in hybrid vehicles (both a combustion engine and electric power), alternative fuels (including compressed natural gas, ethanol, and methanol),fuel cells (greater efficiency, less pollution), electric cars, and computer-controlled roadways to move traffic more efficiently. Development of improved batteries to power electric cars and lighter, more efficient fuel cells than are currently used are two technologies that are receiving considerable attention and hold the most promise to replacing fossil fuels with less polluting fuels.

Burning significant amounts of natural gas, ethanol, or methanol in conventional engines would help circumvent such economic and political problems as the trade deficits and the political instability that come with burning petroleum products. Natural gas is more abundant than once thought, and it burns much cleaner than gasoline or diesel fuel. Ethanol can be produced from a variety of crops and plant material. It is mandated that 10 percent ethanol be blended with gasoline in many states. Methanol (or wood alcohol) can be synthesized from coal or natural gas, and it can be used with a conventional fuel tank.

Fuel cells, which combine fuel with oxygen electrically, hold great promise for the future. They are more efficient and less polluting than combustion engines. They are most efficient with straight hydrogen and oxygen. Electric cars had once been competitive in the early twentieth century, and major automobile makers began introducing new electric vehicles in the 1990s. General Motors’ EV1, for example, was put on the market with considerable advertising fanfare in 1996. On the plus side, power stations that generate electricity are more efficient, and can better control pollution, than millions of gasoline-powered vehicles. However, lower gasoline prices caused automakers to postpone their entries into the electric car market. In the first decade of the twenty-first century, increases in gasoline prices and concern about greenhouse gases in the environment renewed the interest in electric cars. As batteries increased in efficiency, electric cars became increasingly practical for many consumers. By the third quarter of 2024, electric vehicles made up 8.9 percent of vehicle sales in the United States.

Another approach to greater vehicle efficiency is building automobiles with less mass. More plastics are now used in nonload-bearing applications. For load-bearing applications, the use of stronger steel and more aluminum cuts mass.

Flying

Aviation began at the turn of the twentieth century. At first, the type of heavier-than-air craft flown by Wilbur and Orville Wright in 1903 hardly seemed to be competition for the lighter-than-air dirigibles developed by Ferdinand von Zeppelin, first flown three years before. Dirigibles, which need virtually no power to remain airborne, need less energy than airplanes to move a cargo. However, airplanes move much faster, so an airplane can make several trips while a dirigible makes one. Eventually that advantage, coupled with some spectacular dirigible crashes, nudged investments toward airplanes.

The gas turbine, or turbojet, engine allowed large-scale transatlantic passenger flights to begin in the 1950s. Propeller-driven planes (and seaplanes) had provided limited service, but jets cut flight times sufficiently to avoid the need for sleeping berths. By the 1990s, commercial jet airplanes were more than twice as efficient as planes built in the 1950s because of hotter turbines, better aerodynamics, lighter construction materials, and the efficiencies of larger “jumbo” jets. Experts believe that aircraft efficiency can double again. Many argue that it must, because flying is so energy intensive.

Astronautics, Electronics, and the Future

Flight outside the is especially energy intensive. It is also very costly because the launch vehicles are either partly expendable (as with the space shuttle) or entirely throwaway. For the near future, the only profitable payloads in space will be equipment for communications, navigational aids, weather monitoring, and mapping. Liquid-fueled rockets, essentially jets carrying their own oxygen, were developed in World War II and the Cold War, and finally found significant commercial payloads in the 1990s. Rockets have increased in power per unit mass and dropped in cost. Airplanes are being developed that can switch by degrees from air-breathing to rocket mode. As with dirigibles and airplanes, entirely new technologies may evolve. Spaceflight will expand, but the degree to which this will happen is a matter of considerable conjecture and debate.

On Earth, however, although the near future points to a global growth in energy consumption for transportation, it may actually begin to decrease at some point. Computers, communication satellites, and fiber optics allow people to work at many jobs at home (or anywhere), and even business meetings can be “attended” from a great distance. Virtual travel and telecommuting are convenient, cause no congestion, and use relatively little energy. Constellations of communications satellites and improved data compression are steadily lowering transmission costs. The telecommuting workers who are now a small fraction of the labor force will eventually grow to a significantly larger fraction who can live and work nearly anywhere. The General Services Administration of the federal government in the United States has a goal to have 50 percent of its employees telecommute at least one or two days a week. If telecommuting can be implemented on a broader scale, it could trigger a worldwide diffusion of population and may encourage larger movement than the car-and-truck move to the suburbs from central cities.

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