Wednesday, October 30, 2013

A Rural Explanation for VMT Decline?

Vehicle miles traveled (VMT) have been declining in the US, Japan and Europe. There has been a lot of interest in what is causing the US shift, and much of the interest is in whether shifting demographic preferences have caused the decline, especially among young people. Less attention has been given to geographic differences, specifically urban and rural distinctions. The Brookings Institute made note of these distinction in their 2008 report The Road...Less Traveled. Yet it seems this distinction may deserve more attention than it is getting. The figure below shows the total VMT for the US plus for urban and rural travel up through the year 2011.


You can see the overall decline in VMT that starts in 2007 after the peak of over three trillion miles. The green line shows urban travel, which stopped increasing and leveled off, and the red is rural travel, which started to decline in 2002. At a glance it seems that much of the decline in overall VMT can be attributed to declines the rural road networks. This does not preclude demographic shifts, but people haven't been moving as much in the US and there hasn't been a dramatic rural to urban migration starting in 2002. Potentially some of the rural decline could be from land getting reclassified from rural to urban, but I don't expect this would be a big effect.

Here is a table that shows the data from the above figure along with the relative changes from peak VMT for rural and urban areas. Where rural VMT has dropped 13% from the 2002 peak, urban VMT has dropped less than one percent from the peak in 2007. That the relative share of VMT has increased in urban areas is just an artifact of the rural decline. Travel reductions are not evenly distributed.

Rural and Urban Vehicle Miles Traveled in US 2002-2011
Rural Urban Total  % Rural % of Rural Peak % Urban % of Urban Peak
1.1274 1.7281 2.856 39.48% 100.00% 60.52% 86.64%
1.0844 1.8058 2.890 37.52% 96.19% 62.48% 90.54%
1.0684 1.8964 2.965 36.04% 94.77% 63.96% 95.08%
1.0324 1.9757 3.008 34.32% 91.57% 65.68% 99.06%
1.0371 1.9772 3.014 34.41% 91.99% 65.59% 99.13%
1.0353 1.9945 3.030 34.17% 91.83% 65.83% 100.00%
0.990418 1.9831 2.974 33.31% 87.85% 66.69% 99.43%
0.98218 1.9746 2.957 33.22% 87.12% 66.78% 99.00%
0.984148   1.9824   2.967   33.17%   87.29%   66.83%   99.39%
Miles in trillions

The decline in rural VMT is partly because rural areas are associated with so much more travel than urban areas. States that have higher shares of urbanization has lower VMT per capita than the national average. Using data from the FHWA I calculated the correlation between the percent of the population that is urban and VMT per capita at -.58, which is a fairly strong association between increased urban population and lower VMT. 

Certainly there are changes afoot in the US economy and transportation. This post is not intended to make any dramatic claims about VMT declines. However, these data suggest that the bulk of decline is from rural reductions, not urban reductions. Of course, we need more research about this.


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