All but one lane was closed Wednesday on northbound Interstate 5 in Seattle, just south of I-90, where two cars rolled over.
Washington State Patrol Trooper Rick Johnson said on Twitter that the crash was caused by a driver who is in custody, citing DUI as a factor.
The backup extended for miles, he said. All lanes have since reopened.
Johnson said the driver who caused the collision was driving an SUV when he changed lanes, striking a smaller car and sideswiping a third vehicle. Both the SUV and the first car hit rolled over. The passenger in the SUV suffered significant injuries and the driver of the car, an elderly woman, suffered non-life-threatening injuries, Johnson said.
Because of the injuries, police are processing the scene as an assault, Johnson said.
Johnson first tweeted about the crash just before 11 a.m. Wednesday.
The left three lanes reopened around 12:15 p.m., and all lanes reopened around 12:40 p.m., but Johnson said people should still avoid the area.