To avoid a conk to the head, Uncle Tom says to hug the cliff edge if the face is nearly vertical. If the cliff face is sloping, rock will most likely bounce and roll, ultimately landing toward the edge of the road nearest the cliff face, then bounce into the middle of the road. The above drawing is Dave's opinion - "It's a crapshoot," he says. Google wasn't helpful. When I was hiking earlier this year, we actually saw a rock fall from a vertical cliff face - it landed about 10 ft into the middle of the road. This seems to validate Uncle Tom's guidance. Isn't there a mathematical model to quantify risk? Anyone?