Human Mobility Is Not Random, Physicist Discovers

Barabási, along with co-authors Marta C. González and César A. Hidalgo, studied the trajectory of 100,000 anonymized cell phone users – randomly selected from more than 6 million users – and tracked them for a six-month period. They found that contrary to what the prevailing Lévy flight and random walk models suggest, human trajectories show that while most individuals travel only short distances and a few regularly move over hundreds of miles, they all follow a simple pattern regardless of time and distance, and they have a strong tendency to return to locations they visited before.

“We found that human trajectories show a high degree of temporal and spatial regularity, each individual being characterized by a time-independent characteristic travel distance and a significant probability to return to a few highly frequented locations, like home and work” said Albert-László Barabási, Distinguished Professor of Physics and Director of the Center for Complex Network Research (CCNR) at Northeastern University.

“Our study shows that humans, after only three months of saturated behavior, reach stability in their mobility patterns, and the trajectories become identical,” added Marta C. González, Ph.D. in Physics and Research Assistant at the CCNR. “People devote their time to a few locations, although spending their remaining time in five to 50 places, visited with diminished regularity.”

The location of cell phone users was located every time they received or initiated a call or a text message, allowing Barabási and his team to reconstruct the user’s time-resolved trajectory. In order to make sure that the findings were not affected by an irregular call pattern, the researchers also studied the data set that captured the location of 206 cell phone users, recorded every two hours for an entire week. The two data sets showed similar results, the second validating the first.

The findings of this research complement the notion that human mobility can be generalized by the Lévy flight statistics, as suggested by a 2006 study that found that bank note dispersal is a proxy for human movement. That study analyzed the dispersal of about half-a-million dollar bills in the U.S. and concluded that human travel on geographical scales is an ambivalent and effectively superdiffusive process. By using a different methodology, Barabási’s group was able to find evidence to complement those findings.

“Contrary to bank notes, mobile phones are carried by the same individual during his/her daily routine, offering the best proxy to capture individual human trajectories, said César A. Hidalgo, Ph.D. and Research Assistant at the CCNR. “Also, unlike dollar bills that always follow the trajectory of the current owner and diffuse, humans display significant regularity and do not diffuse.”

“The inherent similarity in travel patterns of individuals could impact all phenomena driven by human mobility, from epidemic prevention to emergency response, urban planning, traffic forecasting and agent-based modeling,” added Barabási.