CHP officers say a driver pulled his big rig onto a shoulder near Westside Boulevard so he could check his tires.
Another driver drifted off the road and smashed his semi into the first big rig just before 6:30 a.m.
The crash started a fire and seriously injured the driver in the second 18-wheeler.
CHP officers had to pull him out of his rig and a medical helicopter rushed him to a Modesto hospital.
They say the other driver and a passenger did not suffer any injuries.
The slow lane is closed for now and officers expect it to last until past noon to give clearance for recovery and cleanup.