Using Flickr Geotags to Map the World's Cities

You'll also notice a bit of color coding on the maps. Apparently, Fischer was able to guess that the picture taker's mode of transportation--presumably using the time stamps and distance traveled between a user's pictures. He then created a color code: Black is walking (less than 7mph), Red is bicycling or equivalent speed (less than 19mph), Blue is motor vehicles on normal roads (less than 43mph); Green is freeways or rapid transit. — fastcompany.com