I've come in at Chiang Khong / Huay Xai before and road on 3 to Nateyu to get on 13 down to Luang Prabang, This is the route Google maps shows. If I use Kurviger they show a more direct route.
Attached is the Google map, Google route in blue, I'd dr
Attached is the Google map, Google route in blue, I'd dr