Demographic component to traffic volume

energyecon has a nice chart DOT Miles Driven: Overlay 2007-2011 that compares each of the yearly trendlines. There could be a demographic element to the 2011 decline as mortality for people 50-70 is higher than for 30-49 cohort, so some drivers are leaving the pool permanently. The youngest baby boomers are 47 so the boomer group as a whole will be a drag on the miles driven metric.