For six decades, Americans have tended to drive more every year. But in the middle of the last decade, the number of miles driven — both over all and per capita — began to drop, notes a report to be published on Tuesday by U.S. Pirg, a nonprofit advocacy organization.

People tend to drive less during recessions, since fewer people are working (and commuting), and most are looking for ways to save money. But Phineas Baxandall, an author of the report and senior analyst for U.S. Pirg, said the changes preceded the recent recession and appeared to be part of a structural shift that is largely rooted in changing demographics, especially the rise of so-called millennials — today’s teenagers and twentysomethings. “Millennials aren’t driving cars,” he said.

In fact, younger people are less likely to drive — or even to have driver’s licenses — than past generations for whom driving was a birthright and the open road a symbol of freedom. Research by Michael Sivak of the Transportation Research Institute at the University of Michigan found that young people are getting driver’s licenses in smaller numbers than previous generations.

Online life might have something to do with the change, he suggested. “A higher proportion of Internet users was associated with a lower licensure rate,” he wrote in a recent study. “This finding is consistent with the hypothesis that access to virtual contact reduces the need for actual contact among young people.”