No, they don't. The game has physical barriers specifically to prevent this. That's, again, literally the point of these inaccessible areas. They're sympathetic landscape areas - such as mountains, forests, the sea - that block any further progress to keep you on the map.
This is Forza Horizon. They don't scan real places. The maps are interpretations of the locations in which they're set, with significantly shorter distances between locations, smaller locations, and sometimes a mild rearrangement of geography.
And again, it's literally the point that they don't have to recreate the entire (four main islands, not one) nation of Japan.