The woman was killed after stopping to help another driver, who had been involved in a single-car crash on Interstate 880 in Fremont.
It happened just after 2:30 a.m. on the off-ramp leading to Highway 262.
California Highway Patrol says she was in a car that pulled over to help move the crashed vehicle to safety on the side of the road.
Investigators say she was standing between the two cars when a man driving a Ford truck plowed into the back of her car.
It was pushed into her and she was killed.
Police say the driver of the truck ran away and is still at-large.
The driver she was originally trying to help was later arrested for driving under the influence.
The CHP says the identity of the victim has not been released, pending notification of next of kin.