Transportation - Energy and the Environment - page 189

160

200

120

Fuel
consumption

150

80

100

Vehicle
registrations

40

50

1920 1940 1960 1980 2000

Year

Figure 8.1 The number of registered highway vehicles and their annual fuel consumption in the United
States since 1920. (Data from the National Highway Traffic Administration. 1999.)

The rapid growth in the number of U.S. highway transportation vehicles in the twentieth
century is graphically depicted in Figure 8.1. The population of road vehicles in use has grown at
a rate of about 3.4 million vehicles per year since 1950, 1 reaching 201.5 million in 1995, of which
64% are automobiles, 36% are trucks (light, medium, and heavy duty), and 0.3% are buses. 2 The
number of these vehicles is closely approaching the number of eligible drivers, but the annual rate
of increase in vehicles exceeds that of the U.S. population increase by one million per year. 3
The growth in the number of highway vehicles, mostly privately owned, has been accompanied
by a growth in highways and roads, publicly financed and maintained. In the United States there are
currently 3.93 million miles of highways (79% rural, 21% urban), about 40 vehicles per highway
mile. In the last decade, highway miles have increased at an annual rate of about 0.1% per year,
much less than the 1.7% per year growth in the vehicle population. This slow growth in highway
miles reflects both the difficulty of siting new roadways, especially in urban regions, and the scarcity
of public funds for infrastructure improvement. The concentration of national vehicle ownership
in urban areas (which accounts for only 21% of highway miles but 75% of the population), along

1 This amounts to a current annual growth rate of 1.7 %. The world vehicle population, about 600 million with
production of about 50 million new vehicles per year, is growing at an annual rate of 2.2 %.
2 About 15.4 million new vehicles are sold each year, 12 million of which are needed to replace the aging
members of the 201.5 million vehicles in use. The approximate vehicle life expectancy, equal to the ratio of
vehicle population to new vehicle sales, is 13 years.
3 The United States has the highest ratio of vehicle to human population of any nation. Most developing
countries currently have vehicle/human ratios about the same as that of the United States in the 1920-1940
period.