The motorcyclist, later identified as 38-year-old William Westmoreland, was speeding south on the freeway just after 3 p.m. when he crashed into the back of another car near 112th Street in South Everett, according to Trooper Mark Francis.
"He's injured, he's bleeding, and people go to help him out," Francis said. "One woman in particular who was helping out, he tried to carjack her."
But the woman and her husband, who had three children in the car with them, successfully fought off the man, who then ran west from the freeway.
The husband tells KOMO News he chased after the man after he ran across all four lanes of southbound I-5, and then climbed up the adjacent embankment.
The husband continued pursuing the man through a thorny thicket, over a barbed wire fence, and then over a higher chain link fence, but lost him as he ran between some houses in the adjoining residential complex.
Troopers then got a report minutes later that a silver and black Mazda Miata was stolen from a nearby residential area, but they're still investigating whether it was connected.
Francis said Westmoreland left his identification at the scene. He said the Stanwood man was wearing a red jersey with the No. 35 on it. If you have any information, you're asked to call police.