Packet forwarding in Public Transport Networks is particularly challenging due to the high mobility, rapidly changing topology and intermittent connectivity observed in these networks. Though clustering of nodes can aid forwarding decision in these Delay Tolerant Networks (DTNs), the clustering process is extremely costly in a large network. In this paper, we introduce a generic efficient clustering method which is suitable for grouping the nodes of large networks. We also demonstrated how encounter frequencies of public transport networks can be fed to that clustering algorithm in order to build clusters of nodes. And finally, our large scale extensive simulation study on real bus traces shows the efficacy of clustering in packet forwarding.