The road network graph of Europe contains more than 100 million nodes and half a billion edges. Performing route searches on such a graph in a setup with limited memory, like a Personal Navigation Device or a mobile phone, is a non-trivial task. For this purpose, modern maps are partitioned and stored in blocks (map tiles) according to a geometrical grid. Therefore, the major factor during the computation is the number of blocks of graph data loaded into memory. An A*-search algorithm with strong lower bounds for shortest path distances is important to optimize the number of cache loads.
Precomputed lower bounds for shortest paths between two tiles can be used to modify the order in which nodes are settled and further speed up Dijkstra-like searches. The right use of these properties will help to speed up route search on embedded devices and allow to build applications with a lower memory and energy footprint compared to the ones available today.
