Humans Everywhere Have the Same Commute

Whether it's chasing down antelope on foot or driving to work to earn money for Seamless, daily travel has always been an intrinsic part of humans' lives. But there are only 24 hours in a day; is there a limit to how much commuting we can feasibly do?

Venetian physicist Cesare Marchetti posited exactly that in 1994, and published a paper that found that despite technological progress and shifting economic constraints (deer hunting vs. paper pushing, for example), humans have tended to commute for a total of around an hour a day throughout history. That finding became known as Marchetti's constant, which has since morphed into one of those strange bits of trivia that produces endless internet detritus.

But the thing about Marchetti's constant is that researchers can't quite agree on whether or not it's constant at all. Part of the problem is that it's rather difficult to track just how long large populations actually commute for, especially throughout history. Previous estimates have required exhaustive calculation of transit times based on city, demographic, and infrastructure characteristics. But new research published to the arXiv pre-print server used a novel, modern method: Its data were collected by tracking commuters' cell phones in three countries.

The trio of researchers from MIT collected anonymous mobile phone signaling data—which tower a phone is connected to can provide location info—from telecom providers in the Ivory Coast, Portugal, and Boston, which was then used to develop commute times for people using any method of transportation. An additional data set, based on GPS data from Milan, is only based on driving times, and thus isn't fully comparable to everything else in the report.

The team, consisting of Kevin Kung, Stanislav Sobolevsky, and Carlo Ratti, was able to draw a number of interesting conclusions. For example, it seems pretty standard that during the day (defined as 8 AM to 8 PM in this study), people tend to move around a lot, while at night they tend to stick to their most popular places.

They also found that, regardless of city, people tend to have commute distances of about the same length:

And as you might expect, people that have longer commutes tend to leave home and work earlier:

But what about, as the arXiv Blog put it, the Universal Law of Commuting? To be of any value in solving a question with so many variables, the data must be precise. The authors argue that location data from cell phones connecting with towers may not offer as much resolution as something like GPS, but "a mobile phone is typically carried by an individual throughout the day and thus accurate tracks the mobility pattern on an individual level, and is widespread enough in terms of adaptation—even in developing countries—that it allows us to adequately sample the country-wide population (unlike taxi, Foursquare, or GPS traces)."

In other words, because phones can serve as a location tracker for a huge swath of the population—who also carry those devices far more consistently than, say, a GPS—they offer a very good picture of how people are commuting. The results are pretty striking:

Regardless of distance of commute, folks in each location tended to have a very high probability of commuting for the same amount of time. (Milan is all over the place because the GPS data is only for cars.) That's pretty shocking when you think about it; no matter where people live in a region, their commute is likely to take about as long as everyone else, regardless of whether they're walking, riding, or driving.

While it varied slightly location to location, it's also surprising just how similar the mean commute time really is. And if you add them all up, they do seem remarkably close to the hour commute Marchetti first posited.

What does it mean? The authors cite work by Mokhtarian and Chen that suggested commuters have a fairly universal "time budget." In other words, because there's only so much time we can spend commuting each day, commute times tend to be similar regardless of distance. For an extreme example, if you were offered a job 50 miles away, you'd probably only take it if the commute was reasonable—say around an hour. And in the city, congestion and public transport combine for a longer commute time despite the smaller distance traveled.

Naturally, the authors note that more study is needed to come up with a definitive answer, as data from three regions around the world isn't representative of all humans' commutes. We're capable of some truly epic traffic jams, remember. And having lived in Los Angeles, São Paulo, and now New York, I can say commuting for an hour total per day is a dream I've yet to realize. (Or maybe I'm more tolerant of wasting life sitting in traffic?)

But in testing a new method for transit data collection, the study was a success. And, really, whether you call it Marchetti's constant or a time budget, the concept is intuitive: Because we can only travel so fast, and are only willing to spend so much time traveling, the average commute time is likely to drive to a similar mean. Now, whether or not prehistoric humans had similar constraints as they commuted to their favorite foraging spots is a question that still needs an answer.

@derektmead