I'll be interested to see the traffic counts on this.
There is thru traffic on 101, of course, but the highway is also two lanes almost immediately north of Willits, and I would guess a lot of the traffic came from Willits itself or the Brooktrails development, northwest of town (which is almost as populous as Willits). If that's the case, then that traffic is not going anywhere.
Plus, with growing weed being legal in the rest of the state now, Mendocino is going to see a lot less traffic leaving the county.
It seems like the Olancha bypass is more necessary, even though a lot of the bitching there is from Angelenos trying to keep an 80mph clip coming back from Mammoth and Yosemite.