Rural Interstate highway volumes versus loading over time. Source: FHWA
This chart, from the Federal Highway Administration’s 2011 Highway Statistic series (most recent year available) shows the changes in both rural Interstate highway traffic volume and loading since 1970. Loading here refers to the weight of vehicles. The 2002 peak in volume is noted on the chart (Urban Interstates peaked in 2007 per FHWA figures), but notice how loadings continued to increase, even through the 2008-09 recession. This suggests that, even with the peak in overall vehicle travel, truck volumes and/or truck weights have continued to increase. This is important to consider in light of Interstate pavement conditions and, on some corridors, capacity concerns.