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From Growth to Decline: Vehicle Miles Traveled in Kentucky and the USA

May 27, 2010

In a previous post I contrasted Kentucky’s energy plan with Ireland’s. Both plan to grow biomass to offset some of their fossil fuel use, but Ireland consumes far less fossil fuel than Kentucky, so can offset a much larger proportion of its energy use with each bale of switchgrass. Ireland is planning to further reduce its energy use, while Kentucky is planning for growth. My analysis of traffic volume trends leads me to believe that Kentucky’s transportation growth forecasts will not materialize.

The transportation sector is the biggest contributor to Kentucky’s projected growth in energy use and greenhouse gas emissions between now and 2030 (Figure 1). The Kentucky Transportation Cabinet has projected 2.2% annual growth in vehicle miles traveled between 2008 and 2030.

Figure 1. Historical and reference case projections for gross emissions growth in Kentucky by sector, 1990-2030 (million metric tonnes of carbon dioxide equivalent basis). Figure from Strait et al., 2010 (draft). Click image to load source document.

Figure 2. Annual change in vehicle miles traveled in Kentucky, 1983-2009, with linear regression projected to 2030. Figure presented to the Kentucky Climate Action Council's Transportation and Land Use Technical Working Group. Click image to load original presentation and links to data sources.

It’s an unlikely projection, given Kentucky’s recent history. I plotted the observed change in vehicle miles traveled for each year between 1983 and 2009 (Figure 2). There were many years in the 1980s and ’90s in which growth exceeded 2.2%, but there hasn’t been a year like that in the past decade. Recent years have been a mixture of modest growth and pronounced decline. If the trend continues then Kentucky is headed for a future in which we drive less each year, not more.

Figure 3. Annual change in total and per capita vehicle miles of travel (VMT) in Kentucky, 1983-2009. Figure presented to the Kentucky Climate Action Council's Transportation and Land Use Technical Working Group. Click image to load original presentation and links to data sources.

Figure 4. Per capita income and vehicle miles traveled in Kentucky, 1983-2009. Income figures are not adjusted for inflation. Figure presented to the Kentucky Climate Action Council's Transportation and Land Use Technical Working Group. Click image to load original presentation and links to data sources.

The trend is even more pronounced if we consider miles traveled per person (Figure 3). In only three of the past 10 years did individuals in Kentucky drive more than the year before. Kentucky’s recent history is characterized by individuals driving less each year.

Before 2000 almost every increase in personal income was accompanied by an increase in travel. That correlation reversed a decade ago (Figure 4). After decades of growth, per capita travel declined gradually through most of the past decade, and fell precipitously in 2008, while nominal income continued to grow. The average person in Kentucky drove less in 2009 than in 1996, but made 60% more in nominal income.

The Kentucky Transportation Cabinet’s projection of 2.2% annual growth translates to 67% more vehicle miles traveled in 2030 than in 2005 (Figure 5). Projection of the linear regression of recently observed annual changes in travel results in a 41% decline over the same period. The Kentucky Transportation Cabinet’s projection exceeds the Energy Information Administration’s annual projections for light vehicles and trucks (1.5 and 1.7%, respectively). If individual driving habits don’t change in Kentucky between 2005 and 2030 then we can expect 20% more travel by 2030, based on projected population growth.

Figure 5. A range of projections for vehicle miles of travel in Kentucky until 2030. Figure presented to the Kentucky Climate Action Council's Transportation and Land Use Technical Working Group. Click image to load original presentation and links to data sources.

Which of these projections will come closest to reality? I don’t have a crystal ball, but my money’s on the bottom one. The Energy Information Administration has a history of overstating growth projections, and population growth has exceeded any observed increase in travel for most of the past decade. As fuel prices rise I expect people to drive less each year, as individuals have already been doing for most of the past decade. Kentucky’s projected increase in vehicle miles traveled looks to me like a shaky foundation on which to build an energy plan.

Figure 6. Annual change in vehicle miles traveled in the USA, 1983-2009, with best fit regression line. US Department of Transportation data.

The national trend in vehicle miles traveled looks similar to Kentucky’s (Figure 6). The downward slope is a little gentler, but the regression line fits better. The nation may be a year or two behind Kentucky as we move from annual growth to annual declines in travel, but it looks to me like decline is the wave of the future. Annual growth in vehicle miles of travel has been below the ~1% population growth rate in recent years, meaning that most Americans are already driving less each year, not more. Vehicle miles traveled were just 0.2% higher in 2009 than in 2008, but this was sufficient for the Bureau of Transportation Statistics to issue a factsheet last month titled “Upward Trend in Vehicle-Miles Resumed in 2009.”

Back in 2005 Daniel K. Simon placed a $400 Long Bet that vehicles in the USA would travel less in 2010 than in 2005.  It looked like a risky proposition until the outlier year of 2008, when vehicle miles traveled fell by 3.4%; now his bet looks pretty safe. Increases in 2006, 2007, and 2009 were all well below 1%, and there was 0.7% less traffic in the first quarter of 2010 than in the first quarter of 2009. Vehicle miles traveled would have to increase by 1.2% in 2010 to reach 2005 levels. I will be surprised if they do.

Even though he’ll probably win, I think Simon’s bet was a little premature. The outlier year of 2008 was a lucky break for him. A similar bet for the period 2010 to 2015 looks like a much safer thing.

So why is Kentucky predicting growth, when long term trends suggest we’re headed for decline? I suspect the answer has more to do with politics than careful analysis. Projecting a high growth rate makes for a goal that’s easy to beat. In Kentucky’s case, the governor is claiming that business as usual will lead to a 40% increase in energy consumption between 2005 and 2025, but the state’s efficiency and conservation measures can hold growth down to 15%. It’s nice to have some wiggle room when you’re billing increasing energy use as conservation.

Another motivator may be the Clean Air Act, which cuts federal transportation funding, and imposes other consequences, on regions that fail to attain emissions targets. Once again, projecting high emissions sets the bar low so that it will be easy to clear.

Finally, growth is often perceived as a goal in itself. Our society appears to work on the implicit assumption that continued wellbeing depends on our ability to consume ever more. Since projections of decline are unpopular and politically unpalatable we continue to plan for growth and ignore the harbingers of decline.

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2 Comments leave one →
  1. August 8, 2010 2:40 pm

    I dont think its correct to compare Ireland’s far lower fossil fuel consumption, mainly due to the much lower amount of cars and industry compared to mainland USA

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  1. Energy: Driving less, using more for food « Bluegrass reVISIONS

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