The California Highway Patrol said the 26-year-old Richmond woman was driving on the McEwen Road onramp to eastbound Highway 4 near Martinez when her car veered down the embankment.
The crash occurred just after 3 p.m. Saturday.
The highway patrol said she was not "properly restrained" and was ejected from her Nissan.
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She died at the scene.
It's unclear what caused the crash or whether alcohol or drugs were a factor.
No other cars were involved in the crash and no one else was hurt, the CHP said.
Those who witnessed the crash are asked to contact the Contra Costa CHP at (925) 646-4980.