SEATTLE -- Troopers are crediting a Good Samaritan in helping catch a woman suspected of running over a pedestrian, then fleeing the scene early Friday morning.

The State Patrol says a female driver was getting off I-5 near 6th and James around 8:45 a.m. when she hit a male pedestrian, throwing the man several feet in the air.

The driver sped off, back down I-5 South. She then struck a parked semi in Tukwila and was soon arrested by troopers.

The victim, 51-year-old Michael Lindsay, was taken to Harborview Medical Center where he is in critical condition -- and doctors fear the man will not survive.

But if not for a Good Samaritan, troopers think the woman would have gotten away with it.

"The witness did a text book job, superb job," said Trooper J.P. McAuliffe.

That witness, who wants to remain anonymous, dialed 911 and followed the driver, who police identified as Rose Dorsey of Seattle.

Investigators say Dorsey drove through Seattle and then got back on I-5. Troopers arrested her when she slammed into the back of a parked semi on the freeway's shoulder in Tukwila. Troopers believe Dorsey was driving under the influence -- that's something they wouldn't have known if they didn't catch her shortly after the original crash.

"There's certain aspects of the case we never would have had," McAuliffe said.

The witness who saw this unfold was shocked to see the pedestrian hit and the driver keep going, especially knowing the pedestrian hit the front windshield, right in front of the driver's face.

Dorsey told investigators her windshield just exploded.

"It's in my opinion that it's obvious something happened at the very least you should stop and investigate what caused the damage to the vehicle instead of continuing on your merry way," McAuliffe said.

Dorsey is being held on investigation of DUI and felonious hit and run.

Troopers say Lindsay was a homeless man who was on his way to get some medication.