|Distance of Zurich residents to their friends.
from: Axhausen, K.W. and A. Frei (2008)
But the thing that struck me most was the time scale of the links. A recent paper in PNAS looked at bus trips in Singapore. Kay and colleagues drew a line between people if they travelled on the same bus on the same day. In the figure above the red lines show a shared bus trip on Monday, the greens are trips together on Tuesday and a cyan are trips on Wednesday. The last picture are all the weekday trips put together. By Friday, all the people are now connected through shared trips.
It turned out that even by Wednesday all Singapore bus travelers were connected. There are just three days of separation between bus travelers.
A bus trip isn't enough time to make friends. But it is enough time to check out what other people are wearing, what mobile they are using, what newspaper they are reading and even to overhear a few opions about the world. It is enough time for all sorts of interesting social information to spread. This result would have appealed to Stanley Milgram, whose small-world study first led to the 6-degrees idea. Cities fundamentally change the individual's social world in many different ways, not least in the time it takes for us to connect to each other.