Ride-Hailing ETA & Routing System Architecture Questions

System design and architecture questions modeled on large-scale ride-hailing and on-demand mobility platforms: how a production-grade ETA (estimated time of arrival) and routing system is built and scaled. Covers real-time driver and GPS telemetry ingestion, map-matching (point-to-segment snapping, Hidden Markov Model approaches), road-graph representation and geospatial indexing (H3, S2, quadtrees), routing algorithms (Dijkstra, A*, contraction hierarchies) versus ETA prediction models, live traffic-data integration, caching and API contract design, multi-region and geo-distributed deployment, fault tolerance, and data consistency trade-offs at rideshare scale. Questions reason about the general system-design problem faced by this class of platform, not any single company's non-public internal implementation.