If that fellow passenger on the bus or train looks familiar, it is probably because you have seen him or her before.
For the first time, researchers here have mapped out patterns involving such "familiar strangers" meeting at the same time and place each day.
Critically, such patterns may hold clues to how diseases can spread in an epidemic.
Researchers at the Future Cities Laboratory, located in the National University of Singapore University Town, analysed anonymous ez-link smart card data for a single week's worth of bus trips.
That worked out to more than 20 million bus trips taken by some 2.9 million commuters.
The researchers at the laboratory, which was set up by ETH Zurich and the National Research Foundation, were especially interested in repeated "in-vehicle encounters", in which two people were on the same bus at the same time.
There were 18 million such pairs, repeated roughly once every 24 hours, or at the same time every day. Eighty-five per cent of these repeat encounters occurred in the morning, rather than in the afternoon, explained Mr Sun Lijun, an NUS doctoral student at the Future Cities Laboratory.
Why? "You go to work around the same time every day but you don't always leave at the same time," he said.
On average, a commuter has a one in three chance of encountering the same people the next day, Mr Sun added.
The work is not published yet but has been uploaded on the manuscript open-access site Arxiv.org.
The concept of familiar strangers is not new. The noted psychologist Stanley Milgram once took photos in major train stations during rush hour and asked commuters to point out people they recognised but had never spoken to. On average, they recognised four strangers.
The work by Mr Sun and his colleagues has several key uses.
One, it makes for more accurate simulations of how individuals move and behave in space and time. People are not evenly dispersed but they do cluster and move in sync.
Second, such patterns can indicate how diseases might spread from person to person.
It may also suggest how people become friends and form social links. As you encounter the same friendly face again and again, especially people whose similar habits become synchronised with yours, you are more likely to chit-chat or say hello, Mr Sun said.
Project manager Vaidehi Shah, 27, commutes by bus from Marine Parade to her Tanglin office each day. "I see familiar faces about three to four days a week; more in the morning than in the evening," she said. But she has not spoken to anyone yet.
She said: "I wish there was some sort of visible sign such as a sticker or badge" for commuters to show that they were willing to chat. "Then commutes might be transformed from something to be endured, to an opportunity for engaging conversation."
