by Michael Sivak.

This is the latest in my series of analyses on the relationship between road transportation and economic activity in the United States. Of interest in this study were the changes in distance driven per inflation-adjusted GDP (gross domestic product) since the end of the Second World War.

For each year from 1946 through 2017, distance driven by all vehicles was obtained from the U.S. Department of Transportation, while GDP in chained 2012 dollars came from the U.S. Bureau of Economic Analysis. (Several values in each set were recently updated.) The results are shown in the chart below.

Distance driven per inflation-adjusted GDP increased from 1946 until 1958, and then declined through 1966. Thereafter, this measure increased until plateauing from the early 1970s through the early 1990s, and then decreased through 2017.

The absolute maximum—235.7 miles per thousand dollars—was reached in 1976. This value was up 42% from 165.6 miles per thousand dollars in 1946. The 2017 value—177.4 miles per thousand dollars—was 25% lower than the maximum.

The most noteworthy finding is the decrease in distance driven per unit of economic activity since the early 1990s. A possible explanation for this pattern would be an increased proportional contribution to GDP from activities that do not require road transportation. Indeed, there are several lines of supporting evidence for this hypothesis:

• GDP attributed to data processing, internet publishing, and related services increased by a factor of 11.5 from 2000 to 2017.

• GDP attributed to truck transportation as a percentage of the total GDP decreased from 1.0% in 2000 to 0.7% in 2017.

• Retail e-commerce sales as percentage of all sales increased from 0.8% in 2000 to 8.9% in 2017. (However, a recent analysis did not find a decrease in the amount of time people spend traveling using all modes of transportation in connection with purchasing goods and services between 2007 and 2017.)

Michael Sivak is the managing director of Sivak Applied Research and the former director of Sustainable Worldwide Transportation at the University of Michigan.