Caltrans is closing all lanes of traffic on Highway 101 in southern Petaluma Friday night as part of the Kastania Road overcrossing construction project at the South Petaluma Boulevard off-ramp.

Northbound traffic will be blocked from 11:59 p.m. Friday to 7 a.m. Saturday. As a detour, traffic will exit at a temporary off-ramp to Petaluma Boulevard South and return to northbound 101 at the Petaluma Boulevard South on-ramp.

Southbound 101 in the area will be closed from 11 p.m. Friday to 6 a.m. Saturday and will involve a longer detour. Drivers will have to exit at Highway 116/Lakeville Highway, take Highway 37 west and continue to southbound 101.

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