Caltrans press release:

On Thursday, October 9, traffic on US Highway 101 on the south end of the Willits Bypass Project will be diverted to a long-term detour.  In order to tie the detour into the existing highway, one-way traffic control with a pilot car will be in effect from 2AM 4AM to 9PM on Thursday only.  Motorists should anticipate major traffic delays which could reach one hour or more during peak traffic times, with southbound traffic on US Highway 101 backing up into the northern end of Willits.  Motorists are advised to leave early to allow for extra travel time, or to alter their travel plans to avoid this section of highway.  This work is being done during the day due to nighttime temperatures which are too cold for paving.

The detour will eventually become the southbound onramp to US Highway 101 from Route 20.  Motorists are urged to drive with caution through the detour which contains a curve with a 20 MPH advisory speed.  The detour will remain in place until the bypass is completed in summer 2017.