If you do a bike trip or bike tour with Adventure Cycling Assoc, they issue you one of these triangles and require that you have in on your back or the back of your bike.
About four years ago I started using it for my bike tours that are not with ACA too. On a hazy rainy day, that thing really stands out. And on this trip one day I put it on my left pannier instead of centered on the dry bag on top of my rack, perhaps I imagined something that was not there but I got the feeling that traffic gave me a little more room with the triangle on my left pannier.
But nothing stands out in fog, but the traffic was not going very fast either so this was safer than it looks.
But at least in weather like that, none of the drivers felt that they were so good that they could send texts to their friends, their eyes were on the road.