As the title states, anyone planning this route in the near future, the road behind Shiga Kogen on the Kusatsu Onsen side is blocked due to volcanic activity since August. However google is still showing the bus route as active despite the buses only driving until Shiba Kogen from the Nagano side.
I booked my hotels in July before it got blocked, so I wasn't aware of this and I expect that others won't notice either because google still shows the buses as if they were running normally. I only noticed due to a sign mentioning the road closure at Nagano station.
If you're staying in Nagano City or are planning to travel to Nagano City, the route via Takasaki isn't much longer, but it doesn't go past the snow monkey park. If you plan on staying in Shibu Onsen near the snow monkey park. You will have to do a large detour to get around the mountain.
So unless you don't mind a 5 hour detour around the mountain like I had to do today instead of the normal 2 hour trip, I'd avoid staying the night in Shibu Onsen or closeby if your plan is to cross the mountain to Kusatsu Onsen.
by Alternative-Ask20