The higher-resolution frequency surely comes from people who have Google Maps open while they drive, for directions.
Also, slow traffic is slow. If you're stuck in gridlock, even if your location only updates once every 5 minutes, that might just be 5 blocks. Average that across a quarter of all vehicles and that should be plenty of data for block-level traffic estimation.
Also, slow traffic is slow. If you're stuck in gridlock, even if your location only updates once every 5 minutes, that might just be 5 blocks. Average that across a quarter of all vehicles and that should be plenty of data for block-level traffic estimation.