Investigators said they found three bullet holes in the passenger side of the car, but are still looking into the circumstances that led up to the shooting.
"All we know is that this poor guy got shot while he was driving on the freeway," said Lt. Ian Hebert, of the Contra Costa County Sheriff's Office.
Dispatchers with the sheriff's office received several 911 calls at about 3 p.m. of a shooting on Highway 4 near Bailey Road, sheriff's officials said.
Callers said one vehicle shot at another.
The investigation shut down eastbound CA-4 east of San Marco Boulevard for nearly six hours.
"Because it's such an extended crime scene -- about 300 yards -- we had to walk it. There were casings in the roadway and things that would have been destroyed if we let people continue to travel on it," said Lt. Hebert.
Sheriff's officials said the driver is not being identifying now.
An investigation has closed eastbound Highway 4 and the highway remained closed until about 9:00 p.m. according to the sheriff's office and the California Highway Patrol.
Thousands of drivers got caught up into backup.
"I sat in traffic for two and a half hours," said Lee Anderson, of Bay Point. "Sitting in traffic bumper to bumper. People were cutting up the sides trying to cut through the traffic. It was just insane!"
Sheriff's officials said they have no suspects.
Sheriff's officials are asking anyone with information about the incident to get in touch with their office at (925) 646-2441.