Greyhound managed transportation options on the grounds of the 1939 New York World's Fair. (Other side of the pamphlet also available). Photo by A.Currell, used under Creative Commons licensing.Over the past century, the structure of our cities has changed dramatically, but why? That's the question developer and researcher Christopher Leinberger tackled in The Option of Urbanism: Investing in a New American Dream. Walkable urbanism is on our mind as our intrepid editor Heather attends the New Partners for Smart Growth conference, where Leinberger is speaking, so we thought we'd share this time-traveling look at cities.