The 64-year-old driver was in the No. 2 lane of southbound I-680 at about 2:25 p.m. when, for an unknown reason, her vehicle swerved to the left and struck the center divide just south of the Oak Park Boulevard overcrossing, Yox said.
The 1999 Miata overturned multiple times before coming to rest on its roof, Yox said.
The convertible was not equipped with roll bars and although the woman was wearing her seatbelt, she suffered major injuries, including a cervical spine fracture and scalp avulsion, Yox said.
She was taken to John Muir Medical Center in Walnut Creek. She is expected to survive her injuries.
Although there were several witnesses to the crash, no other vehicles were involved in it.
CHP Officers are still trying to determine the cause of the crash, but have determined that the woman was not under the influence of alcohol or drugs.