of new aircraft (measured by energy consumed per seat-mile flown) declined by 60 percent during the first 40 years of jet travel. The authors estimate that 57 percent of this decline stemmed from increases in engine efficiency, 22 percent from increases in aerodynamic performance, 17 percent from increased load factors, and 4 percent from operational changes such as flight time efficiency (that is, reduced time on the ground or in noncruise portions of the flight). They anticipate energy efficiency improvements of 1–2 percent per year for the next two decades, yielding a total improvement of more than 30 percent over this period. The Federal Aviation Administration expects air travel demand (in passenger emplanements) to grow about 3 percent per year over the next several decades.15
This presents a major challenge to efforts to reduce fuel consumption in this sector, because energy efficiency per passenger emplanement is expected to improve by only 1–2 percent per year (Lee et al., 2004). This energy efficiency improvement will not be enough to counter the expected growth in demand.
The United States spends about 6–7 percent of its gross domestic product (GDP) on the movement of freight. According to the Federal Highway Administration (FHWA, 2007), about 21 billion tons of freight were moved in 2006 (including 4 billion tons in pipeline movements).16 The FHWA expects U.S. freight transport to continue to grow by 2 percent per year over the next two to three decades as the economy grows and domestic and international trade increases, resulting in an 85 percent increase in freight tonnage by 2035 (to 37 billion tons). Factoring in 0.5 percent annual growth in energy efficiency in the freight sector means that total energy use for freight movement will grow by 40 percent or more.
Table 3.8 shows projections of freight tonnage by mode for 2035. These projections are based in large part on assumptions for GDP growth, as freight volumes have historically tracked economic growth.
Trucking dominates freight shipment in the United States in terms of both tonnage and shipment value (on the latter measure, it accounts for 95 percent of shipments). The dominance of the truck mode is not expected to change during