Starting at 10 p.m. tonight, Oregon Department of Transportation crews will close the Interstate 5 southbound off-ramp to Interstate 405 (exit 302B) overnight for barrier replacement.Attenuator barriers are installed at freeway splits and exits to reduce the severity of crashes and protect the concrete barriers behind them. The attenuator barrier at the I-405 southbound split to the top deck of the Fremont Bridge is hit often. Crews will replace the attenuator with a model that cost less to repair. Replacing the attenuator requires an overnight closure of the ramp from 10 p.m. on Friday, Nov. 19, to 8 a.m. on Saturday, Nov. 20. Detour:Motorists traveling to I-405 from I-5 southbound will continue on I-5 southbound and take the Marquam Bridge to I-405 northbound.