It happened about 12:40 a.m., near where the highway crosses the Stillaguamish River.
Investigators also believe the woman had been involved as a driver in a separate collision about a mile away shortly before she was struck, said trooper Keith Leary with the Washington State Patrol.
The woman was identified Tuesday as Tara R. Olson.
Police believe Olson had been driving a 2003 Ford minivan that collided with another vehicle at the intersection of 236th Street NE and 19th Avenue NE, just north of the river near Arlington.
She reportedly drove away from the scene of that crash. She was southbound on the highway when she got out of her van and began walking for an unknown reason, Leary said.
Olson was struck by a Ferndale man, 26, driving a Toyota Celica, police said. She died at the scene. The Ferndale man was not injured.
The crash remains under investigation.
Troopers believe the woman may have been drinking, but they were waiting on toxicology results before making a final determination, Leary said.
Rikki King: 425-339-3449; email@example.com
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