Was it simply that economic growth was so fast in the 1990s? Perhaps. But that doesn’t seem the most likely explanation. The gap between G.D.P. and miles driven continued to grow last decade, when economic growth was mediocre. And the rapid economic growth of the 1960s did not outpace the increase in driving.

The Center for Clean Air Policy — a Washington group that advocates for walkable cities, public transportation and other so-called smart growth policies — released the chart at a briefing on Capitol Hill on Wednesday. I asked Steve Winkelman, the center’s director of transportation and adaptation programs, what he thought explained the divergence of economic growth and driving growth. Excerpts from his reply follow:

… a couple of months back I took a quick look at relative economic growth in sectors that I guessed were less travel intensive (data limitations hamper assessment of the VMT intensity of specific economic sectors). It is interesting note that from 1998-2008, knowledge- and service-oriented economic sectors such as information, finance, real estate and health care were responsible for more than two thirds of GDP growth, while extracting, manufacturing, transporting and selling physical goods generated less than one third of GDP growth over that period. In the previous decade, these more physically-intensive sectors contributed more than half of all GDP growth…

Transportation planners have been predicting saturation in travel for decades, for example once women fully penetrate the workforce. Perhaps that saturation is finally happening. The big demographic trends are aging of the baby boomers, increasing numbers of households without children and increasing proportion of minority and immigrant households, who typically have lower travel. We’ve also seen strong growth in transit ridership: up 38% percent since 1995, vs. population growth of 14% and highway VMT growth of 21%….

The jury is still out on the net impact of telecommuting and e-commerce on travel demand. While the number of telecommuters increased from about 3 million in 1993 to 6 million in 2008, that’s still only 4% of work trips, and work trips are only about a quarter of all VMT. In fact, VMT for work has decreased from 1969-2009, but shopping VMT almost quadrupled. So, while Amazon.com and Netflix are changing the way we’re shopping and entertaining ourselves, there’s a sense that the internet can both substitute trips but generate others….

The center’s report, arguing that building more roads is not the best way to produce more economic growth, is available on its Web site.