All westbound lanes of the busy 10 Freeway in Fontana were shut down during rush-hour Thursday after a big rig slammed into an overpass bridge at Citrus Avenue.
Traffic was backed up for about three miles, the California Highway Patrol said.
The accident was reported shortly after 5 p.m. Initially, only one lane was closed, the CHP said. But all lanes on the busy east-west artery were closed around 5:30 p.m.
The Sierra Avenue onramp to the westbound 10 also was closed.
Traffic was being diverted off at Citrus Avenue. There were no immediate reports on whether anyone was injured.
The CHP said it was unclear how long the lanes would be closed.