The tale of China's modernization is being told along its winding dirt trails, the paved gray roads in its cities and, more recently, its ambitious network of highways. In rural Hainan, farmers still pedal to their paddy fields before dawn, bumping along paths of dusty red soil. In the urban explosions of Beijing and Shanghai, however, bicycles now have to fight for space with ever-increasing numbers of motor vehicles—and residents have to fight to breathe.