“Perhaps young people are too happy at home checking Facebook. In a study of 15 countries, Michael Sivak, a professor at the University of Michigan’s Transportation Research Institute (who also contributed to the D.M.V. research), found that when young people spent more time on the Internet, they delayed getting their driver’s licenses. “More time on Facebook probably means less time on the road,” he said. That may mean safer roads, but it also means a bumpier, less vibrant economy.”