If there was a car found on the overgrown road, it could be possible that it leads to the Pala Ferry. This would be a reasonable explanation for the VW bus tipped over with the deceased DHARMA workman "Roger".
Either the road was never completed, or it was completely overgrown before Radzinsky began making the blast door map, which doesn't show any roads.
It looks like the "road" ends where there was a concrete wall in the Swan. Maybe it is a tunnel, not a road.
But if it was a tunnel, it wouldn't need the switchbacks.