Automobiles and suburbs, in an M.C. Escher-like way, created each other. But the rapid urbanization of America, led by Millennials, may erase the car. Newly released U.S. census data show that, for the first time in a century, cities are growing faster than surrounding suburbs. And that, tied to other demographic and psychographic trends, could pose potentially serious challenges to automakers desperately seeking further growth. It doesn't help that Millennials are also less enamored of cars than previous generations.