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The world longest road
(feddit.org)
I believe that goes through the Road of Bones in Siberia. From what I saw of Long Way Round (where Ewan McGregor and Charley Boorman ride around the world on motorcycles), it's not really a road much of the year. You wouldn't want to walk it in the winter for obvious reasons. You also wouldn't want to walk it in the warmer seasons, because the snow melts and floods the road.
It's only a road in the sense that Google Maps marks it as one.
The documentary simply couldn't continue on the planned route, and they hucked it over to Alaska early on. That was after putting up with some shit ass mud in and around Mongolia where they were dropping their bikes every 10 feet (and I am not exaggerating).