The most popular place to put a city park is, increasingly, on a highway. Cities looking to boost their downtowns, or to improve downtrodden neighborhoods, are creating “highway cap parks” on decks constructed over freeways that cut through the urban center. Pittsburgh, Philadelphia, Denver and Dallas have deck parks underway. Atlanta, Houston, Minneapolis and Santa Monica, California, are among the cities considering similar projects. In crowded cities, highway deck parks are a way to create new acreage and provide green space that can spur downtown development. Capping a highway to create a park also can reconnect urban neighborhoods sliced apart...