The absolute number of vehicles reached a maximum in 2008. However, it is likely that this was only a temporary maximum and that the decline after 2008 was primarily driven by the current economic downturn that started in 2008. Consequently, with the improving economy and the expected increase in the U.S. population, it is highly likely that (from a long-term perspective) the absolute number of vehicles has not yet peaked.On the other hand, the rates of vehicles per person, licensed driver, and household reached their maxima prior to the onset of the current economic downturn. Consequently, it is likely that the declines in these rates prior to the current economic downturn (i.e., prior to 2008) reflect other societal changes that influence the need for vehicles (e.g., increases in telecommuting and in the use of public transportation). Therefore, the recent maxima in these rates have better chances of being long-term peaks as well.
But Sivak is smart enough to hedge his prediction:
However, because the changes in the rates from 2008 on likely reflect both the relevant societal changes and the current economic downturn, whether the recent maxima in the rates will represent long-term peaks as well will be influenced by the extent to which the relevant societal changes turn out to be permanent.