They don't drive cars, which impacts more than you might think. From Salon.
This is the one change by millennials that will change absolutely everything
Let’s take a look at the era that began in 2001, when the ﬁrst Millennials graduated college, got jobs, and started families. Eight years later, in 2009, Millennials drove 23 percent fewer miles on average than their same-age predecessors did in 2001. That is, their average mileage—VMT, or vehicle miles traveled—plummeted from 10,300 miles a year to 7,900, a difference of 2,400 miles a year, or 46 fewer miles a week.
It’s not that they stopped traveling. While Millennials made 15 percent fewer trips by car, they took 16 percent more bike trips than their same-age predecessors did in 2001, and their public–transit passenger miles increased by a whopping 40 percent. That’s 117 more miles annually biking, walking, or taking public transit than their same-age predecessors used in 2001.
When a cohort of the size of the Millennial generation changes behavior that radically, it’s a little like what happens when a third of the people on board a ferry decide to move from starboard to port: the entire boat starts to list. Which is what is happening to the United States. In every ﬁve-year period from 1945 to 2004, Americans had driven more miles than they did the half-decade before. In 2004, the average American drove 85 percent more than in 1970. But by 2011, the average American was driving 6 percent fewer miles than in 2004. Baby Boomers and Gen Xers were a small part of the reason—they drove somewhat less in 2009 than in 2001—but the big cause was the Millennials. What makes this even more dramatic is that, by 2009, only half the Millennial generation was even out of high school. If all eighty million Millennials retain their current driving habits for the next twenty-ﬁve years, the US population will increase by 21 percent, but total VMT will be even less than it is today, and per capita VMT—the vehicle miles traveled per person—will fall off the table.