Over the last two decades the contribution of transportation to total energy use and CO2 emissions in the USA increased in absolute and relative terms. This paper develops a decomposition scheme which helps to identify the magnitude and the relative effects of the various factors underlying these trends in the US transportation energy use and CO2 emissions between 1970 and 1991. This decomposition scheme has the advantages of simplicity, exhaustiveness, lucidity of interpretation, and intuitive appeal. Its application to US transportation data reveals that the growth in people's propensity to travel, population, and gross domestic product (GDP) were the three most important factors driving up US transportation energy use and CO2 emissions in the 1970–1991 period. The effects of changes in modal structure were smaller, but not trivial. The actual increases of US transportation energy use and CO2 emissions were substantially less than the sum of the effects of the above four factors due to improvements in transportation energy efficiency and decreases in the transportation intensity of GDP. Increases in US transportation energy use and CO2 emissions resulted from developments in freight transportation rather than from passenger transportation in the 1970–1991 period.
