  Although a lot of attentions have been paid to human mobility, the
relationship between travel pattern with city structure is still unclear. Here
we probe into this relationship by analyzing the metro passenger trip
data.There are two unprecedented findings. One, from the average view a linear
law exists between the individual's travel distance with his original distance
to city center. The mechanism underlying is a travel pattern we called "ring
aggregation", i.e., the daily movement of city passengers is just aggregating
to a ring with roughly equal distance to city center.Interestingly, for the
round trips the daily travel pattern can be regarded as a switching between the
home ring at outer area with the office ring at the inner area. Second, this
linear law and ring aggregation pattern seems to be an exclusive characteristic
of the metro system. It can not be found in short distance transportation
modes, such as bicycle and taxi, neither as multiple transportation modes. This
means the ring aggregation pattern is a token of the relationship between
travel pattern with city structure in the large scale space.
