What is a shortcut edge and when is it precomputed?
When a node is removed from the graph, a shortcut edge connects its two neighbours and preserves the shortest path distance. The edge is precomputed offline by iteratively contracting low-rank nodes (intersections with few connections). Highway junctions are contracted last.
Answered in
Why Google Maps Computes Shortcuts OfflineContraction Hierarchies precomputes shortcut edges offline so routing queries skip neighbourhood streets and touch only 2,000 nodes in under 10 milliseconds.
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