Let's give this a go.
Here's a three minute clip from the 700km ride north, probably the most interesting part of the highway where it winds steeply up to the edge of the Chiang Mai basin. The city is in surrounded by mountains, like Kathmandu or Mexico City.
The footage is not great, and my 850's 5-pot sounds like a bloody Chinook through these laptop speakers, but you'll get the idea. You can see how steep it is by how slow the "sip-lor" (10-wheel) trucks are going.