Phones don't keep a GPS fix in the background 24/7. This would be prohibitively expensive, battery-wise. iPhones do have approximate location available at all

Phones don't keep a GPS fix in the background 24/7. This would be prohibitively expensive, battery-wise. iPhones do have approximate location available at all times, via wi-fi scanning/databases and tower ~triangulation. This wouldn't be good enough to infer any sort of acute acceleration/deceleration. All of that said, none of it helps if you were sitting at a red light for 3 minutes before being rear-ended.