I always considered that the best route to Udon is via Lampang -> HIghway 11 to Phitsanulok -> Highway 12 towards Khon Kaen and then up to Udon, either via Khon Kaen, or taking an earlier left near Nong Bua Lamphu.

I just plugged it into Google maps, and the first route it offered was a bit shorter by distance, avoids Highway 12 completely but instead takes me past Chat Trakan, Nakhon Thai and straight through Loei province, like this:

Does the above make sense?

Have done both ways and it seems to work out to be the same time as it is a bit harder to get around slow traffic on the shorter route. I have friends in Udon and they have found the same thing. As it is around the same time we generally go one way and come back on the other route just to give a change of scenery.

Thanks! Any difference in how risky the road is, especially nighttime? And what time do you usually take, roughly? The distance is similar to the drive to Bangkok, but clearly a more challenging road, so I'm thinking it could take 9-10 hours?

The Google map route is cheaper fuel wise and nicer views! The road is narrower but is fine to use and is slightly quicker as it's more direct. Trucking companies (like mine) use this route to Northern Isan.

I have done the trip scores of times and the route you have indicated is the best and fastest I have found. Where the road turns off east of Phu Reua and intersects with Highway 201 south of Loei is at a town called Wang Saphung. That part can be a bit confusing if it is your first time as can be the section from Highway 11 to Chattrakarn. I would count on doing the trip in less than 8 hours in any conditions.

Just did it a couple of days ago on a bike

7 hours including stops