So, one of the biggest questions in planning and development today – in California and elsewhere – is what accounts for the Millenials' preferences for urban living and less driving. Is it generational? Or a lousy economy?

"I think our answer is yes," says Brian Taylor, an urban planning professor at UCLA and head of the Lewis Center for Regional Policy Studies there. 

Taylor is one of many academic researchers – most of them, it seems, based in Los Angeles – who are trying to understand one of the most remarkable trend reversals in American history: the end of growth in driving. Since 2007, vehicle miles traveled (VMT) has been flat, after growing consistently for a hundred years.

This trend reversal coincided, of course, with the biggest economic downturn since the Great Depression. But by most analyses, this reversal began before the big recession began in 2008, and it has continued to persist even though that recession is long past. So what's going on? Are people – especially young people – permanently changing their patterns? Or can they just not afford to drive as much as they used to?

The popular media's narrative is, of course, the first explanation: A new generation of Americans – the Millenials, who supposedly prefer a more urban lifestyle in overwhelming numbers. The truth, apparently, is a little more complicated – as many of the L.A. researchers discussed at the recent American Collegiate Schools of Planning conference. Among other things, the drop in driving may be due not to urbanites who choose not to drive – but, rather, to poor people who can't afford to.

In the one hand, Taylor said, UCLA's researchers have found that there actually is a statistically significant relationship between your age and amount of driving you do (not just the amount you personally drive, but the amount of traveling you do in private autos even if others are driving). People born in the ‘60s – now between the ages of 44 and 54 – tend to drive 5% more than average. For people born in the ‘80s, it's 7% less. And for people born in the ‘90s, it's 16% less.

On the other hand, the actual number of people engaged in the urban millennial lifestyle isn't very high – at least according to research reported by one of Taylor's Ph.D. candidates, Celsie Ralph. After analyzing data about young adults – who she defined as people between the ages of 16 and 36 – Ralph divided the population into five categories: "drivers" and "long-distance trekkers" (who drive virtually all the time), "urbanistas" and "multimodals" (who drive between 50% and 80% of the time), and people who are carless.

What she found was that even among young adults, more than 80% of the population fall into the "driver" and "long-distance trekker" category. Only 6% can be classified as the classic Millenials – the "urbanistas" and "multimodals". But 14% of all young adults have no car at all, and most of them have low incomes.

Ralph said that although the urbanistas and the multimodals are growing in number, "the bigger story is what his happening in lowest income. There's a really dramatic increase in people without cars." These are not necessarily unemployed poor people. Many are working poor who have low-wage jobs.

And while it makes sense that poor residents in cities don't have cars, she and other researchers are finding that they're not the only ones. Poor residents in the suburbs and rural areas often don't have cars either. For example, Ralph found that the number of carless households in low-density residential areas is on the rise. "We see a really remarkable increase in people with fewest resources living in the worst place," she said. "The people in D.C. are doing it by choice but also there are people in rural Ohio who have no choice."

And this growing two-tier structure among people with lower incomes is creating a huge divide in the cost of travel and access to jobs. Another UCLA doctoral student, Trevor Thomas, found a stark relationship between the geographical location within a metropolitan region and the amount of driving the working poor must do.

Thomas compared poverty and VMT in three communities in the Los Angeles area: Boyle Heights, located in East Los Angeles just east of downtown Los Angeles; El Monte, further east in the San Gabriel Valley; and Palmdale, located in the Antelope Valley some 40 miles north of the San Fernando Valley. 

All three are mixed communities with considerable poverty. But their proximity to job centers is vastly different. And so is the relationship between poverty and driving.

Not surprisingly, Thomas found that in Boyle Heights – a neighborhood with a 28% poverty rate located a short bus ride from downtown L.A. – a 1% increase in the community's poverty rate will yield a per-capita VMT reduction of 12 miles per year. 

In El Monte, with a 24% poverty rate but located about 15 miles east of Boyle Heights, a 1% increase in the poverty rate will yield a per-capita increase of 12 VMT per year.

And in Palmdale, which has a 17% poverty rate, a 1% increase in the poverty rate will yield a per-capita increase of 47 VMT per year.

In other words, poor people in the suburbs are more dependent on their cars, probably because they can't take public transit to work.

The actual annual VMT numbers in this last study aren't that large. But the trend is clear: The urbanistas get a lot of publicity but there aren't that many of them. VMT is levelling off in part because there are more people who are carless – but those people are mostly poor. And there's an increasing bifurcation among the poor. The urban poor can survive a downturn because of public transit service, while the suburban poor are chained to their cars just like everybody else – only they have a much tougher time paying their car bills.

There are, of course, other measurements of Millenial behavior besides VMT. Millenials are buying houses and cars – and getting drivers' licenses – at a slower rate than their predecessors. These trends may hold over time; after all, habits engrained at a young age often last a lifetime. Or the urban-style Millenials may simply grow into a suburban lifestyle later than previous generations, as many demographers suggest.

But there's no question that, whatever's going on these days, it's not as simple as urbanistas sitting in coffee shops. The two-tier economy and the growing number of working poor – in both cities and suburbs – is an important part of the trend as well.