How does Google Maps know you're stuck before you do?

As the designated late friend who takes hours to get dressed and is always fashionably behind schedule, I mentally rely on Google Maps' ETA. I check it way before I actually need to leave, lock in that comforting little number (okay, 20 minutes, I still have time), and build my whole schedule around it. Then I finally head out, and the second I do, that number balloons because traffic has appeared out of nowhere.

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