It's hard to imagine that just a few decades ago, people thought jogging in the the US would be a fad. Now you'd be hard pressed to walk a few blocks without seeing someone on a run—which for data nerds begs a lot of questions: Where are people running? Where are they not? Using data from the fitness app RunKeeper, Nathan Yau has created maps of running patterns in several major cities. While the data is limited to those who use the app (and not all runners), the information could help urban planners better direct traffic flow and even use the data to determine what types of environments are more amenable to exercise.