Caltrans plans to close southbound Highway 101 in Petaluma Friday night to demolish the remaining half of the Old Redwood Highway overpass.

All three southbound lanes will be closed from 10 p.m. until 6 a.m. Saturday morning. Two lanes of northbound Highway 101 will also be closed.

During the closure, southbound traffic will exit at Petaluma Boulevard North and turn right, continue to East Washington Street and turn left, and continue to southbound Highway 101.

The southbound loop on-ramp at Old Redwood Highway will also be closed. Both the northbound on-ramp and off-ramp at Old Redwood Highway will remain open during the work.

If the work is delayed due to weather conditions, it will be rescheduled for a later time, Caltrans said.

The demolition is needed to make way for a new $41 million interchange that is designed to help ease traffic across Petaluma. Traffic shifted to the new eastbound overpass in February.