Driving in downtown area, decelerating for stop sign, maybe going 20 mph, when ‘brake!’ illuminated on dash and vehicle abruptly came to a complete stop. It seems the automatic emergency braking system engaged and stopped the vehicle. Seat belt locked and items on seats were thrown to floor. Nothing was in the roadway other than driving over a flat metal road construction plate that was placed in the lane, covering trench work done to the road. Fortunately car behind me had enough room to stop before hitting me. Have driven this route many times including driving over this plate for the last several weeks with no issues until today. I have a sore shoulder and neck from seat belt and jerking motion when car stopped unexpectedly.