The California Highway Patrol was called to the scene at 6:45 p.m., where paramedics were already working on the victim.
He was taken to John Mur Medical Center in Walnut Creek. His injuries were described as not life-threatening.
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Authorities had no information as to whether the gunfire came from another car or a fixed position near the freeway.
The CHP asks that anyone with information about the incident call Officer Adam Lane at (925) 586-1879.