There are roads that are driven by Google or what not before going in the map database, and some roads (or 'roads') that came pretty much from aerial photography and drawn in without much concern as to whether they're drivable.

I worked on GPS navigation for nearly a decade, in the pre-Google Maps days and back then it was a lot like what the original poster described than today.

In anyone's map there should be entry bits for the class of road, and other useful information. But this may not be displayed to the end user, only used to make routing decisions etc.