Secret Network of Familiar Strangers Uncovered

Every commuter knows them: strangers with familiar faces that accompany you day in day out. You know each other without really knowing each other. Perhaps, you might even nod at one another in the morning but don’t even know each other's name. Now, researchers paint a comprehensive picture of this secret network of familiar strangers.

The number of commuters is constantly on the rise and the cities we live and move in are growing bigger and bigger. Although this mobility and the encounters along the way have all been part and parcel of everyday life for quite some time, until now researchers have not really been able to gather any comprehensive information on these commuter encounters. Now, for the first time researchers from ETH Zurich’s Future Cities Laboratory and the National University of Singapore describe the network of encounters on public transport in the Asian five-million city state.

Kay W. Axhausen, a professor at ETH Zurich’s Institute for Transport Planning and Systems, his doctoral student Lijun Sun and their colleagues employed a week’s data to track how often and on which routes people used the buses in Singapore. This was made possible thanks to a smart-card system, where passengers touch the card onto an electronic reader while alighting and descending from the bus and the price of the journey is deducted from their credit. This enabled the researchers to gather data on the bus journeys for around 3 million individuals and ascertain who was on the same bus with whom, how often and for how long.

One big network

Previous studies on social networks focused on contacts where the individuals already knew each other to a certain extent. The new study, however, unveils a network of people who are aware of the existence of other individuals, see them repeatedly but don’t actually know them. The more frequent these encounters, the greater the familiarity and the greater the potential for two such familiar strangers to actually make contact, the researchers assume.

“I found it astonishing how much the number of familiar strangers varies from one person to the next,” explains Axhausen. Moreover, it is impressive to see that these individual networks ultimately merge into a large network. Via two or three familiar strangers, every bus-user is connected to everyone else, says Axhausen.

Infections, fashions, rumours

This data is also interesting from a medical perspective as trains and buses actually constitute places with an increased risk of infection. The secret social network that the researchers describe in the latest issue of the journal PNAS reveals how quickly infections could reach over half the population of Singapore via this route.

A totally different and completely harmless form of virulence exists in fashion trends and behaviour patterns, which can also spread via the network of familiar strangers. Something new tends to be increasingly accepted the more often it is seen on others, as Axhausen explains. Last but not least, rumours and news can also spread via such networks in that you might overhear a telephone conversation on the bus, for instance.

Universal principle?

As the next step, the researchers would like to test whether what they have observed in Singapore also holds true for other cities. For example, Rotterdam also has a smart-card system, which would enable similar datasets to be collected. “It would be exciting to see whether our observations conform to an urban standard or whether Singapore is an exception,” explains Axhausen.

If the extensive network of familiar strangers also proves true in other cities, commuters in large cities, which are often perceived as being particularly anonymous, would have undreamt-of possibilities to cultivate new contacts right at their fingertips. Perhaps these insights will even encourage one or two to step out from the shadows of anonymity and make acquaintances of his or her familiar strangers. -- by Angelika Jacobs, © ETHZ (Swiss Federal Institute of Technology)

Reference:

Sun L., Axhausen K.W., Lee D.-H., Huang X.: Understanding metropolitan patterns of daily encounters. PNAS 2013, doi: 10.1073/pnas.1306440110