The results of the clustering on the basis of the accessibility index and the efficiency index are presented in Table 2 and Fig. 4. The three clusters indicate three general types of urban structural hierarchy: the CBD area (C), the sub-central area (B), and the fringe area (A) (see Fig. 3a). Cluster A, to which more than half of all station areas belong, surrounds clusters B and C with lower density and diversity. It may produce trips to clusters B and C, so it has the lowest accessibility index and efficiency index. Cluster B may produce and attract trips from clusters A and C, whereas cluster C may attract trips from clusters A and B. The difference in the rank of the two indexes between clusters B and C implies that the potential for subway ridership is not fully realized for cluster C.