I've seen where vehicles have gone off the road and down into deep ravines and canyons in our area and they weren't visible at all from above or below and the tangles of brush and scrub oak would make it near impossible to signal with a mirror. In many cases I would think that getting out of the vehicle is out of the question because of steep angles, the precarious possition of the vehicle or injury and a whistle can be ruled out in others because of the inability to blow on it, due to injuries. An airhorn with extra cylinder or a battery operated horn would probably be good to carry with the other devices for all around trouble. Kinda makes a case for keeping the emergency kit in the backseat instead of the trunk don't it?