After sixty years of almost constant increases in the annual number of miles Americans drive, since 2004 Americans have decreased their driving per-capita for eight years in a row. Driving miles per person are down especially sharply among Millennials, America’s largest generation that will increasingly dominate national transportation trends.

But some skeptics have suggested that the apparent end of the Driving Boom might be just a temporary hiccup in the trend toward more driving for Americans. By the time Americans took notice of the decline in driving, the economy was in deep recession. Would economic growth bring back rapid increases in driving?

Doubts about whether the Driving Boom has ended make it easier to postpone choices about transforming our transportation system or enacting reforms that disrupt well-established interest groups.

Forty-six states plus the District of Columbia witnessed a reduction in the average number of driving miles per person since the end of the national Driving Boom. North Dakota, Nevada, Louisiana and Alabama are the only states in the nation where driving miles per capita in 2011 were above their 2004 or 2005 peaks. Meanwhile, since 2005, double-digit percent reductions occurred in a diverse collection of states: Alaska, Delaware, Oregon, Georgia, Wyoming, South Carolina, the District of Columbia, Pennsylvania, Indiana and Florida.

The fifty states plus the District of Columbia offer a useful natural experiment to examine different factors behind America’s reduction in driving since 2004. Examining the commonalities and differences in driving trends among states can provide insight into the potential causes behind the downturn in driving and the direction of future trends. While a number of factors will influence the amount of driving in any given state, to the extent that differences in driving trends among the fifty states correlate with differences in the severity of the economic downturn, then the economy could be seen as the dominant factor and future driving trends could be expected to follow the economy as well. In that case, a return to faster economic growth might likely lead to a rapid increase in driving. If instead the extent that differences between states’ trends in the amount of driving reflect other kinds of persistent factors or can’t be easily explained, then we can expect the slowdown in driving to persist even if the economy speeds up.

This study finds that declining rates of driving do not correspond with how badly states suffered economically in recent years. On the contrary:

Among the 23 states in which driving miles per person declined faster than the national average, only six saw unemployment increase faster than the nation as a whole.

Among the 10 states with the largest declines in driving per person, only two rank among the ten with largest increases in unemployment.

Among the 23 states where driving declined faster than the national average, only 11 saw faster-than-average declines in the employed share of their working-age population.

Among the 10 states with the greatest reductions in the employed share of population, only two were also among the ten states with the largest reductions of driving (Georgia and the District of Columbia).

Looking at which states reduced driving most sharply after 2004, there are wide differences which are not easily explained by a single factor:

Average vehicle miles declined in all regions, with large variation within each region. The smallest declines took place in the North Central region, mostly as a result of per person driving increases in North Dakota, and the Gulf South, perhaps influenced by Hurricane Katrina.

Changes in the extent to which state populations live in urban areas do not seem to correspond with changes in driving miles.

The degree to which telecommuting or other arrangements in each state have encouraged people to work more from home does not seem to correspond to changes in the average amount of driving per person.

The evidence suggests that the nation’s per-capita decline in driving cannot be dismissed as a temporary side effect of the recession. While certainly a contributing factor and an economic rebound could be expected to have some upward lift on driving, the recession does not appear to be the prime cause of the fall off in driving over the past eight years. Nor is it clear that future economic growth would lead to a resumption of the postwar Driving Boom. Policy makers can stop wondering whether American driving trends are changing. They should focus carefully on these trends, and start adapting policies to match them.