When I saw it I wondered What could that sign possibly mean? I don't believe that I've ever see such a traffic sign before. But I didn't think much about it. I just snapped my picture and continued on. Now, however, in retrospect, the meaning is obvious: Stop sign ahead. The sign is at the top of a hill facing traffic that is about to go down the hill. Drivers are being warned to get prepared for the stop. However, if the red octagon actually had the word "stop" in it, then the sign would likely be interpreted as a stop sign. And that doesn't make any sense at all at that point.