The man had stopped his Dodge Ram on the shoulder just south of the Sierra Point Parkway on-ramp to inspect his truck, which was having a mechanical problem, CHP Officer Shawn Chase said.
He was standing in front of the pickup when a man driving a Toyota Camry veered onto the right shoulder and slammed into the back of the Dodge Ram, pushing it forward into the man, Chase said.
The man was thrown about 75 feet, and was pronounced dead at the scene, he said.
Chase said CHP officers who responded learned that the driver of the Toyota Camry had been reported missing in San Ramon and was said to be possibly suicidal.
He said it is not clear whether that driver veered off the roadway intentionally or accidentally.
The impact from the Toyota Camry caused the gas tank of the Dodge Ram to rupture, and firefighters have responded to contain the gas spill, Chase said.