The roar peaks. It is the coalescing of centuries of progress, of millions of hours of ingenuity and design. Those who travel in America know the sound of it. Those who hear this roar are progressives, conservatives, Democrats, Republicans, the thoughtful, the thoughtless, the apolitical, the disillusioned, the angry and the hopeful. They are, each of them, dependent on all that has gone before and all that is under way.