Do these two charts explain “peak car”?

January 29, 2014 at 5:55 am CST

Last week we talked about how maybe everyone should put the brakes on any sort of declaration of “peak car.” It’s not to say that we haven’t reached the “peak” but that there are underlying factors that need some exploration. To refresh: “Peak car” is a hypothesis, based largely on some great research out of the University of Michigan that “Per person, per driver, and per household – we now have fewer light-duty vehicles, we drive each of them less, and we consume less fuel than in the past.”

The series of papers (currently four parts) doesn’t really get into any causal data about what might be leading to that decline. That leaves journalists and pundits to draw conclusions on our own. The common assumption is that it’s a mix of underlying trends: Millennial 20-somethings who don’t want to own cars, more restrictions on teenagers who want to get a license, technologies that enable us to work from home and socialize from home, and economic factors.

All of those things are likely correct to some extent, although hard to test. My theory is that a demographic trend that isn’t usually mentioned might turn out to be the most important.

The most recent report out of the University of Michigan is focused on households that don’t have a car or light truck. The report looks at national trends as well as trends in the 30 largest U.S. cities.

Carless households increase as single-person households increase.

It stands to reason that if there’s an increase in the proportion of households that don’t have a car, there will be a related decrease in driving overall, so this is an important component to look at. The research shows that the number of households without a vehicle has been more or less on the uptick since at least 2005 when the Census Bureau began releasing annual figures.

What else has been on the uptick over that time period? Single-person households. Nationwide, the correlation between the growth in single-person households and carless households is very high. Looking at the 30 largest U.S. cities (as the U. of M. report does), we see that pattern reinforced. As the percentage of single-person households increases, so does the percentage of carless households. That last correlation is nearly one-to-one. Only two cities had increases in single-person homes without an increase in carless homes (Austin and Philly).

Why does it matter to driving that single-person households have been rising? Because when you look at household size and vehicles owned, there’s a huge difference between households with one- and two-or-more people. In short, very few homes add a second person without adding a vehicle. There are lots of reasons why that might be the case but one obvious contributor is that in today’s reality of dual-income households, the likelihood that one of them doesn’t live close to work increases.

Percent of carless households by number of occupants.

Who are the single-person households? The majority are senior citizens who have either divorced or outlived their spouses. Another 17 percent are under age 35. The big question, which we touched on last week , is will those Millennials stay in single-person households? And what will happen to the 36 percent (that’s 21 million!) Millennials living at home ? They likely have access to their parents’ car now. But as they strike out on their own, eventually they might need a car of their own.

As the Millennials continue aging – 12,000 turn 30 every day – and moving into new life-stages like marriage (or at least cohabitating) and having kids, I think there’s reason to believe that some of these peaks might just start to reverse themselves.

On the other hand, the number of single-person households has been growing for decades. It could easily continue to increase – as the Baby Boomer cohort ages and 20-somethings stay single longer, in which case “peak car” trends could accelerate.

At the very least, I think it’s early to say that any of these trends are set in stone.

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