Staying indoors: a challenge

After traveling to a few big cities in Asia, like Hong Kong and now Singapore, you pretty quickly notice a common theme: every city is basically one enormous shopping mall. There are an unimaginable number of giant shopping malls, and they're usually connected directly to subway stations. So in theory, you hardly ever need to go outside, you can just jump from one mall to the next via the subway and never bother setting food outdoors.

So that got me thinking, how much of the planet could you cover this way? Most of these cities are also connected by their subways to the airport, so once you've covered as much of one city as you can, you could take the subway to the airport, connect directly to the terminal, then to a jetway onto the plane, and still never set foot outside, and then repeat the process in another city, and then another.

So the question is, how many miles of the earth, and in how many cities, could you cover by moving from covered structure to underground train to airport, without ever setting foot outdoors? You could probably actually compute this given the right data, and possibly even with Google Maps data since it now covers most shopping malls as well as subways.

I just submitted this at http://what-if.xkcd.com/ to see if he's interested in actually figuring it out!