A google image search for “triangle sign” shows that the triangle, on a sign, could mean a lot of things but almost always refers to something ahead that you need to be cautious of. Some of these signs are icons (a little train for a train), some are verging in indexes (maybe the explanation point?) but they are not very symbolic. If I take a triangle out of the road sign panoply and put it on another road sign, it might be indexical to something. The widespread use of the triangle for this context may render the triangle as un-symbolizable, because it will always be iconic of the indexical reference to danger, until civilization ends, everyone forgets this, and different signs, indices, and icons emerge.