On January 26, 2006 Dirk Brockmann, a theoretical physicist and computational epidemiologist at Northwestern University in Evanston, Illinois, L. Hufnagel, and T. Geisel published "The scaling laws of human travel," Nature 439 (2006) 462-65.
Using statistical data from the American currency tracking website, Where's George?, the paper described statistical laws of human travel in the United States, and developed a mathematical model of the spread of infectious disease.
[By January 31, 2009, Where's George? tracked over 149 million bills totaling more than $810 million. (Wikipedia).]