State troopers say the bizarre incident happened Thursday at about 9:45 a.m. near the I-405 interchange with NE 85th Street.
Witnesses told the State Patrol that the woman leapt from the moving car and landed on the pavement. An oncoming semi truck slammed on its brakes and almost jackknifed as it avoided hitting her.
After bringing his rig to a stop, the semi driver ran to the woman, helped her out of the roadway and blocked the lanes with his truck. His truck was hit from behind by a van, but no one in the van was injured and the van was drivable after the impact, said Trooper Julie Judson.
When troopers arrived at the scene, soon afterward, the woman was initially thought to have only scrapes and cuts. But further evaluation found more serious injuries, and she was taken to Harborview Medical Center.
She was identified as a 44-year-old Woodinville woman. The State Patrol says it has no immediate explanation for why she jumped from the car.
An investigation is continuing.