Some people won't see you whatever you do. This Land Rover was protecting a scene at night on a lit, quiet motorway when the Sierra hit it from the back, notwithstanding all the blue lights, etc.
Writing as one who has extensive experience of walking on motorways and putting out signs to protect scenes, I would prefer to put out triangles behind me if I were broken down on the hard shoulder. Even if it alerts a sleepy driver by the bump and clatter as it's driven over, it's better than the first alert being the collision with the back of my car. (That is how most crashes on the hard shoulder occur, but there is no way that "hundreds" die on the hard shoulder every year.)
Writing as one who has extensive experience of walking on motorways and putting out signs to protect scenes, I would prefer to put out triangles behind me if I were broken down on the hard shoulder. Even if it alerts a sleepy driver by the bump and clatter as it's driven over, it's better than the first alert being the collision with the back of my car. (That is how most crashes on the hard shoulder occur, but there is no way that "hundreds" die on the hard shoulder every year.)