  While the individual travel implicates the trace of individual mobility
decision, group travels signify the possible social relationship behind these
traces. Different from online social network, spatial interaction between
individuals is a critical yet unknown dimension to understand the collective
behaviors in a city. In this paper, based on over 127 million trips in Beijing
metro network, we develop a method to distinguish the group travel of friends
from the encounter travel of familiar strangers. We find travels of friends are
among the most predictable groups. These identified friendships are interwoven
and form a friendship network, with structural properties different from
encounter network. The topological role of individuals in this network is found
strongly correlated with her travel predictability. The overall time savings of
about 34190 minutes after redistribution of inefficient group traveler with
flexible travel purposes shows the potential of designing specific traffic
fares for group travel. Our identification and understanding of group travel
may help to develop and organize new traffic mode in the future smart
transportation.
