The handy thing about "peak car" as a concept is that it can nominally be proven in many ways. You’ve got Peak Driver’s License. Peak Registered Vehicle. Peak Gas Consumption. Peak Miles Traveled. There are peaks per person, per household, per demographic. Then you've got your absolute peaks when you add up all of our vehicles and miles together, as if we were all cruising the highways at the same time.
The point of all of this is that any one number is a little dubious, especially in light of that inconvenient economic recession. But Michael Sivak at the University of Michigan Transportation Research Institute has been methodically slicing the question every which way. And the totality of the picture he's built is starting to look pretty convincing.
Earlier this summer, Sivak released data showing that the number of registered light-duty vehicles in America (cars, pickup trucks, SUVs, vans) had peaked per person, per licensed driver and per household in the early to mid 2000s, before the onset of the recession. Because the U.S. population continues to grow, he predicted that the absolute number of vehicles had not yet peaked. But per person and household, we seem willing now to own fewer of the things.
Now he has released a follow-up study [PDF] of how much we drive. As a nation, our total mileage has leveled off (but again, because the population continues to grow, we may surpass this 2006 peak again):
Distance Driven by Light-Duty Vehicles, 1984-2011
More importantly, here is that same data crunched by individual person, vehicle and household, removing the factor of the ever-expanding population:
All of the peaks on that chart occur around 2004, a time that predates both the recession and the housing bust. That means, Sivak suggests, that other factors beyond the temporary state of the economy may be driving these downward trends, from the rise of telecommuting, urbanization and public transit usage to fundamental shifts in the age demographics of drivers. It's possible that miles driven per car alone will tick up as households come to rely on fewer of them, or as more people join car and ride-sharing services (or as we all pile into autonomous cars that never need to park!). But that would be a good sign that we're using vehicles more efficiently.
Put together, these two studies from Sivak suggest that Americans are now driving fewer vehicles than we used to, but also that we're driving each of those vehicles less. "This is an important finding of a double reduction," Sivak writes, "because one does not necessarily lead to the other."