figure 1. Median daily range of cellphone users living in central La, sf, and Ny (darker yellow areas).

the radii of the inner, middle, and outer circles represent the 25th, 50th, and 75th percentiles of these ranges across all users in that area. all maps are drawn to the same scale.

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CDRs are an attractive source of location information for three main reasons: They are collected for all active cellular phones, numbering in the hundreds of millions in the U.S. and billions worldwide; they are already being collected to help operate the networks, so additional uses incur little marginal cost; and they are continuously collected as each voice call and text message completes, enabling timely analysis.

At the same time, CDRs have two significant limitations: They are sparse in time because they are generated only when a phone engages in a voice call or text-message exchange; and they are coarse in space because they record location only at the granularity of a cellular antenna. Not obvious a priori is whether CDRs provide enough information to characterize human mobility in a useful way.

Since 2009, we have pursued a research program aimed at developing sound analysis techniques for exploring aspects of human mobility using CDRs and shown that CDRs are indeed useful for accurately characterizing important aspects of human mobility. Our results to date include the following:

Daily travel. We have determined how far anonymous populations of hundreds of thousands of people travel every day in the Los Angeles, San Francisco, and New York metropolitan areas;

Carbon emissions. We have calculated the carbon emissions due to the home-to-work commutes of these populations, accounting for differences in distance and modes of travel;

Number of workers and event goers. We have identified which residential areas contribute what relative number of workers and holiday parade attendees at a suburban city—Morristown, NJ; and

Traffic volumes. We have estimated relative traffic volumes on the main commuting routes into Morristown.

We validated our results by comparing them against ground truth provided by volunteers and against independent sources (such as the U.S. Census Bureau). Throughout our work, we have taken measures to preserve individual privacy. The rest of this article covers the methodologies and findings of our human-mobility studies based on cellular network data.