Wow. This is really awesome. And beautiful. Cabspotting is a website run by the Exploratorium here in SF, and tracks cabs in the city. I think they describe it best:
The patterns traced by each cab create a living and always-changing map of city life. This map hints at economic, social, and cultural trends that are otherwise invisible. The Exploratorium has invited artists and researchers to use this information to reveal these "Invisible Dynamics."
JUST GO LOOK!