The efficiencies of present-day modes of transportation are reviewed. Future sustainable options are discussed.
Transportation takes about 20% of the energy use worldwide , and this figure is likely to increase. The fact that transportation requires some form of mobile energy storage makes this topic especially challenging for the post-fossil-fuel era. When looking at alternatives, we should realize that nothing matches the energy density of liquid fuels like gasoline or diesel, if the system as a whole is considered. It is, therefore, important to consider the efficiency of various modes of transportation. To assess the possibilities of improvements in efficiency, a brief introduction into the physics of transportation is given first. Subsequently, the efficiencies of present-day modes of transportation—cars, buses, trains, air transport, and bicycles—are reviewed. Finally, new technologies relying on biofuel, electricity, solar power, and hydrogen are discussed.