When a freeway becomes a river, this is what happens. The Cheonggyecheon Freeway used to carry nearly 170,000 vehicles through the heart of Seoul, South Korea, every day. A decade ago, in the face of angry protests, a crusading mayor torn down the elevated 16-lane highway, replacing it with an urban boulevard and park, and “daylighting” the eponymous stream that had been covered by the freeway for nearly 50 years. That mayor, Lee Myung-bak, had once been the president of the company that built the highway, and went on to become president of South Korea. His $900 million urban renewal project ultimately proved to be extremely popular among locals and tourists, reducing pollution and the heat-island effect around the stream, and boosting nearby property values by 300 percent.