Eric Fischer (flickr.com/photos/walkingsf) has a whole series of interesting data visualizations on his Flickr channel. The streams forming the heart shape in the picture below are comprised of taxi routes in SF, using data from Cabspotting.
Start of each trip in green, fading to the end of the trip in purple. Still fabricating routes based on trying to maximize distance to the nearest geotag (origin or destination, in this case). Brightness is the log of the number of routes passing through the point. The geotag avoidance works at least well enough that the street grid is darker than the interior of blocks.
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