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Mobile Information Systems
Volume 9, Issue 1, Pages 37-53

A Safe Exit Algorithm for Continuous Nearest Neighbor Monitoring in Road Networks

Hyung-Ju Cho, Se Jin Kwon, and Tae-Sun Chung

Department of Information & Computer Engineering, Ajou University Woncheon-dong, Suwon Si Yeongtong-gu, Gyeonggi-Do, Republic of Korea

Received 18 January 2013; Accepted 18 January 2013

Copyright © 2013 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Query processing in road networks has been studied extensively in recent years. However, the processing of moving queries in road networks has received little attention. In this paper, we introduce a new algorithm called the Safe Exit Algorithm (SEA), which can efficiently compute the safe exit points of a moving nearest neighbor (NN) query on road networks. The safe region of a query is an area where the query result remains unchanged, provided that the query remains inside the safe region At each safe exit point, the safe region of a query and its non-safe region meet so that a set of safe exit points represents the border of the safe region. Before reaching a safe exit point, the client (query object) does not have to request the server to re-evaluate the query This significantly reduces the server processing costs and the communication costs between the server and moving clients. Extensive experimental results show that SEA outperforms a conventional algorithm by up to two orders of magnitude in terms of communication costs and computation costs.