IIRC, the road was gravel surface at the time. My in-laws lived on a gravel road for many, many years. You don't really see skid marks (if indeed they DID skid). Also, the car traveling in front of them would have raised a significant amount of road dust- probably obscuring the view out the boys' rear-view mirror. They may have not noticed for some time that the girls' car HAD vanished from view. Perhaps the cloud of dust is the cause of the running off the road to begin with- who knows? Finally, spring run-off may have left the creek murky. An upside-down vehicle may not have been noticeable.