The method is remarkably accurate. In a study on 200 people willing to be tracked, the system was, on average, less than 20 meters off when it predicted where any given person would be 24 hours later. The average error was 1,000 meters when the same system tried to predict a person's direction using only that person's past movements and not also those of his friends, says Mirco Musolesi, a computer scientist at the University of Birmingham who led the study.  

 

He cautions that the 200 participants might not reflect the general population—they all lived within 30 miles of Lausanne, Switzerland, and were mainly "students, researchers, and people that are fairly predictable anyway." Even so, he says, the findings were noteworthy because "we are essentially exploiting the synchronized rhythm of the city" for greater predictive insights.