One rock went through a windshield and injured a person, and troopers say several other cars were damaged.
Scott Robinson and Jonathan Katz were driving back to Port Orchard from Utah when a rock smashed through their windshield.
"I thought it was a gunshot. I thought at first I got shot you know, literally," Robinson said.
The men pulled over to the side of the road, and as they were gathering themselves another car was hit with a rock.
Katz, who was the passenger, said he hopes the rock throwing wasn't meant to hurt anyone.
"I'm just hoping it was just some kids that just didn't know, weren't thinking, and it wasn't someone really intentionally trying to hurt somebody," he said.
Robinson has bruises from the rock and was cut by shattered glass.
The State Patrol says the rocks were thrown from the side of the freeway late Saturday night or early Sunday morning.
Anyone with information about the crime is asked to call the Washington State Patrol at (509) 925-5303 and ask for Trooper Dawn Ferrell.