Candidate Sets for Alternative Routes in Road Networks (Extended Abstract)

Introduction and Related Work Routing services have evolved over the past years. Providing only a shortest path is no longer enough. Users expect to be presented with a set of reasonable alternatives. We show how to engineer previous algorithms to provide reasonable alternative paths with better efficiency. Building on these results, we introduce the concept of candidate via nodes to further speed up the computation by an order of magnitude. We show how to perform preprocessing and query variants efficiently. Finally, we provide an extensive experimental evaluation of our method. The shortest path problem can be solved by Dijkstra’s seminal algorithm (Dijkstra 1959). Heuristics to prune the search space provide goal direction and help with scaling. An early goal-directed technique with substantial speedups is arc flags (Lauther 1997; Möhring et al. 2007). The road network is partitioned into regions and each edge stores a flag to indicate if there is a shortest path into a region over the edge. Techniques exploiting the hierarchy inherent to a road network assume that sufficiently long routes enter the arterial network at some point, e.g. a national road or highway. Contraction Hierarchies (CH) (Geisberger et al. 2012) is probably the most prominent of these techniques. The road network is augmentend by carefully chosen shortcut edges while all unnecessary edges are removed. Road networks of continental size can be preprocessed within minutes and (bidirectional Djkstra) queries run in the order of one hundred microseconds. Reconstructing the complete shortest path requires roughly the same time. The fastest CH variant is CHASE (Bauer et al. 2010) that combines CH with arc flags. Its queries run in the order of ten microseconds. (Abraham et al. 2010; 2011a) give analyses of the theoretical performance of speedup techniques to Dijkstra’s algorithm and present an efficient implementation (Abraham et al. 2011b) with query times below one microsecond.