The crash was reported at 7:27 a.m. mid-tunnel on eastbound state Highway 24.
An empty car carrier and a Toyota Scion collided, causing the Scion to overturn, California Highway Patrol Officer Peter Van Eckhardt said.
The crash initially blocked both lanes, preventing emergency vehicles from reaching the scene, Van Eckhardt said.
However, a group of motorists finally exited their cars and pushed the overturned Scion out of the way, clearing one lane, he said.
At about 8:15 a.m. crews were still working to remove the damaged vehicles.
No injuries were reported, and Van Eckhardt expected the crash to be cleared by 8:30 a.m.