Automatic emergency breaking occurred with no warning on a road, that while lanes were closed due to construction, had no observable obstructions including steel plates, etc. the warning occurred a fraction of a second prior to the braking and i only noticed it in retrospect. the car was traveling 25 - 30 mph on a city street that had its right lane being resurfaced. there were no cars immediately in front of me and the car behind me was several car lengths back. there were temporary lane markers divinging the north and south bound lanes. i'm unsure if these are made of metal or plastic but they are no more than 10 in^2.