A group of us rode from Lewiston to Lolo Pass in 2007 and took 12 the entire way because we did not want the climbing of the alternative road (for various reason). The Adventure Cycling warnings for 12 from Lewiston to Kamiah are dead on (pardon the pun). You have to be prepared for narrow roads, big trucks, and high speeds. We were lucky because we rode it early on Sunday morning, so the traffic was less than normal. Even then, we were forced on several occassions to bail out into ditches to avoid big trucks.
The rest of the ride to Lolo is really nice. Unfortunately, for us when we rode it in early June, the entire day was rainy and cold (35 degrees). We were soaked to the bone when we got to Lolo Pass. We woke the next morning to 4" of snow on the ground. Two days before in Lewiston, it was 95-100 degrees. If you areriding in June, be prepared for major weather changes.