Spatial indexing driver matching

Design the spatial indexing layer for a real-time driver-matching system: find the K nearest available drivers to a rider's location within a radius, in a city with up to 100k concurrently moving drivers each emitting a location update every 4 seconds, with p99 match-query latency under 50ms. Drivers' availability flips constantly. Describe the spatial index structure, how you handle the extreme update rate (every driver moves constantly), how nearest-K-within-radius queries run, and the central trade-off.