There are those of us who know enough to identify various deposits on the road and take a cue from that, but anyone who needs an orange triangle to tell them a vehicle is moving slowly probably doesn’t know their s**t well enough to take advantage of the obvious trail.
One could complain this is double trouble. Not only could a car driver come suddenly upon a buggy, but the manure slick behind it would make it harder for the car to stop in time.