I ran into this on my commute just last night. Cop car in the left shoulder, lights flashing. Cop on the right side of the road, waving a flashlight. Oncoming traffic crossing over into my lane and blocking my way. Finally I figured out that the cop was directing traffic and I had to wait until something changed. Eventually the oncoming traffic dwindled, and the cop turned around so I had a harder time seeing the flashlight - I assumed that meant it was my turn to go. I approached very slowly.

I never figured out what the problem was, this was in the middle of the road and not at an intersection. I was more concerned with doing the right thing than trying to determine the root cause.

Google (Waymo?) has been thinking through these exceptions: (around the 9:45 mark) https://www.ted.com/talks/chris_urmson_how_a_driverless_car_...

It should be possible for current technology to (a) detect a cop in the middle of intersaction; (b) stop safely before the intersection and (c) delegate control to a human - possibly someone in the car, possibly a remote one.

The cop can carry a transmitting device which will communicate with cars telling them what to do.