I think you mean Boise, ID. I've traveled US 20 to the US 26 junction, then on to Craters of the Moon NM. The route passes many volcanic features that eventually became Yellowstone volcano. (I've taught geology in Yellowstone) Craters is a very interesting place, take a warm jacket and flashlight if you go into the various lava tubes. You might also plan to stop in Arco, ID on the way and visit the First Breeder Reactor. It's very interesting, very over engineered in a 1950 style.
I've then gone on to Boise using I-84 on US 26. I've not been on the stretch of US 20 between Cary and Mountain Home, I used US 26. I don't see any issues using US 20, I was just following the Snake River Valley on US 26.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI