Two lanes of the freeway remained closed four hours after the collision, and the backup extended for six miles as of 6:30 a.m. The crash scene was cleared by 7:45 a.m., but a four-mile backup still remained from the earlier lane closures.
By 8:45 a.m. there were slowdowns in the vicinity, but the backups were dissipating.
A witness tells KOMO News a semi driver was traveling about 60 mph north on I-5 when it hit another semi truck with a flatbed trailer that had just pulled out of a weigh station south of Exit 142, near Highway 18.
The semi that rear-ended the flatbed burst into flames shortly after the crash. Witnesses say explosions from the semi's fuel tank were about 25 or 30 feet high.
Joshua Roberts and his friend were driving through the area at the time and were the first to run and help the driver who slammed into the back of the flatbed trailer.
"Almost immediately after he rear-ended the gentleman in the flatbed and hit the guardrail, he was immediately out of the truck, kind of wandering around in the middle of the freeway," Roberts said. "(We) ran over, got him off to the side to make sure he was OK."
Cleanup crews worked to remove the semi truck's load of plastic cups before hauling it off.
Investigators say the causing driver faces a speeding violation.