The first Transportation Statistics Annual Report reflected a half-century of growth in the demand for travel, while the last edition reflected stagnation in travel demand over the past half decade (figure 1). The last edition also examined whether recent stagnation of demand is due to temporary economic conditions or a longer term generational shift.
Early Transportation Statistics Annual Reports recognized many contributors to growth in vehicle-miles of travel, including population growth; increased vehicle ownership; increased share of the population with driver licenses; declining number of persons per automobile; increased number of trips per person; longer average trip lengths; and increased vehicle travel by the younger and older populations, by women, and by lower income groups. The reports noted that some factors, such as share of population with driver licenses, were reaching saturation levels and that future growth in vehicle travel might be more closely aligned with population growth. Figure 1 illustrates the growth of several contributing factors to travel, and figure 2 shows the increased share of working age population that contributes to work-related travel.
While the U.S. population has grown steadily in the last two decades, the growth in vehiclemiles of travel (vmt) stagnated after 2007. Figure 3 shows that previous periods of stagnation were not as long, and that public road mileage has remained relatively constant whether vehicle travel was growing or not. The 2013 Transportation Statistics Annual Report notes that the recent decline may be due to economic conditions or to reduced vehicle travel by the younger population. Median household income, a major indicator of economic conditions that affect travel demand, grew steadily until 1999, sagged and rebounded by 2007, and then declined and stagnated (see figure 4) [USDOC CENSUS 2014]. Travel demand is likely to remain flat if income stagnation continues for a large share of the population or if recent travel behavior of the younger population does not change as that cohort ages. Travel demand is likely to return to modest growth if economic conditions improve and if housing and travel choices by Millennials, those born between 1980 and 2000, begin to resemble more closely their Baby Boomer parents, born between 1945 and 1965, as the Millennials enter family formation years.
Housing and travel choices affect more than personal vehicle use. Shifts in housing choices between central cities and suburban communities affect the market for public transit, which has shown recent growth. Although the number of commuters who drove to work increased between 2001 and 2012, the overall share of drive-alone commuters decreased from 88.0 percent to 86.0 percent in that period. Transit riders increased from 4.7 percent of all commuters in 2001 to 5.0 percent in 2012. Anecdotal evidence suggests that travelers are turning more frequently to bicycles, revitalized intercity bus service, new forms of taxicab service, and the availability of vehicle rentals by the hour, little of which was anticipated in the 1990s. The long-term growth in commercial airline travel followed a pattern similar to highway travel, with a decline and rebound in recent years (see figure 5). By 2003 over 80 percent of Americans had flown on a commercial airline [USDOT BTS 2003].
For-hire passenger travel and for-hire freight transportation trends have mirrored each other since the early 1980s, except for the temporary drop in passenger travel following the terrorist attacks on Sept. 11, 2001 (see figure 6). Early Transportation Statistics Annual Reports anticipated continued growth in demand for freight movement, especially with implementation of the North American Free Trade Agreement (NAFTA) in 1994. Freight movement increased through 2007, followed by a decline and rebound that mirrored fluctuations in the general economy (see figure 7).
Early Transportation Statistics Annual Reports questioned if “dematerialization” of the economy through the increasing importance of services over manufacturing and a shift to higher value, lighter goods would affect the demand for freight movement. The increased economic importance of services is evident in employment (see figure 8) [USDOL BLS 2015]. The number of private service-providing jobs grew from 72 to 98 million between 1994 and 2014, while the number of jobs in manufacturing declined from 17 to 12 million. The shift to higher-value, lighter goods is less clear. The value per ton of goods shipped in inflation-adjusted dollars grew only one-tenth of one percent between 1997 and 2012 [USDOT FAF 2015].
While the manufacturing sector experienced a decline in jobs, the tons of goods shipped in the United States increased 1.2 percent per year and ton-miles 0.9 percent per year between 1997 and 2012 [USDOT FAF 2015]. The growth in the tonnage of freight did not keep pace with the economy; 1.485 million tons moved in 1997 per billion dollars of gross domestic product (in 2009 dollars) declined to 1.279 million tons in 2012 [USDOT FAF 2015 and USDOC BEA 2015]. This decline is consistent with the “dematerialization” hypothesis.
The 1994 report, reflecting the early 1990s interest in intermodalism, projected that intermodal freight movements would become increasingly important. Tons moved domestically by multiple modes (plus all mail) increased 4.2 percent per year from 1997 to 2012, reaching 3.2 percent of total tonnage and 11.5 percent of total value in 2012 [USDOT FAF 2015].
The 1994 report reflected a decline in freight movement related to a drop in domestic oil production that has since reversed dramatically (see figure 9). The recent growth in domestic oil production has resulted in increases in the movement of crude petroleum by rail, from 630,000 barrels in January 2010 to 33,706,000 barrels in January 2015 [USDOE EIA 2015]. Localized impacts of this oil boom on population, vehicle travel, and highway fatalities are substantial in places such as North Dakota (see figure 10).