There are roads going upland to the top of the local glacier that are marked as "Impassible" in Icelandic. They really mean it.
When my wife and I were in Iceland, there were stories in the local English-language newspapers giving the story of a French family that drove by such a sign not realizing what it meant. They had to be rescued by a specialized armored vehicle from large rocks being blown off the glacier. Their car was abandoned, totally destroyed.
Joe