The single-engine aircraft landed on the freeway near the Slater Road interchange at about 1:45 p.m.
The plane struck a glancing blow to the roof of a car as it came down, but otherwise landed safely, said Washington state Patrol Trooper Brandon Lee.
He says neither the 47-year-old pilot nor his 48-year-old woman passenger were injured. The occupants of the car that was hit were also unhurt.
Lee says the small plane experienced mechanical issues that cut fuel to the engines just before descending to the Bellingham airport.
The plane then taxied about two miles on the freeway before exiting and parking at an AM-PM gas station and convenience store.
A witness, Steve Garrett, said the plane was just sitting at the gas station when he pulled up.
"I was wondering how it got there," he said. "It must have been an interesting landing."
Garrett said representatives from "every law enforcement agency around" responded to the scene.
"It's a big kerfuffle. The plane seems fine," he said.
The Bellingham Herald reports that the pilot, Tony Dulley, and his girlfriend, Shelby Rush, were flying from Spokane to Bellingham.
Dulley is a plane mechanic who has been flying for more than two decades, Rush said. The couple was coming to Bellingham so he could do mechanical work on planes at the airport when a suspected carburetor issue caused engine trouble seven miles from the airport.
Rush said she was shaken up by the emergency landing and credited their safe landing to Dulley's flying experience.
The Slater Road exit ramp was closed temporarily while the plane taxied along it, but has since reopened, said state transportation officials.