In most so-called megacities with populations over 10 million people, traffic is a megaproblem. And so is the air population. But Tokyo — with a population of 37 million — bucks that trend, thanks to a massive web of public transit that keeps cars off the road. As our transportation correspondent Jeremy Siegel reports, while many countries look to Tokyo as a success story, it may not serve as a model for other parts of the world.