Yau cautions that this data is not complete. It doesn't show all of the running or cycling trips being taken in a city, just those tracked by RunKeeper, and there's also some "noise" in the data -- data recorded in error where, for example, it looks like someone is running on water.
Still, the maps reveal some interesting details. The lines get thicker near bodies of water and on scenic routes, indicating that more people are running that path. Clearly, people prefer to run by the water and in parks, to avoid traffic and enjoy the fresh air and the scenery.
But the maps also give you a sense of how close a city is to scenic routes. In Miami, London and elsewhere, people usually stick to running along the water, whereas in Philadelphia, Chicago and San Francisco, you have far more people running on city streets. In Boise, Charlotte and Minneapolis, you can see that people have fairly easy access to scenic trails nearby.
To see more city maps, check out Yau's site.
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