The incident happened around 6:30 p.m. on Fish Ranch Road near Highway 24, just above the Caldecott Tunnel, near Moraga.
Witnesses say they saw 46-year-old Elnora Caldwell's husband Robert Woods push her out of his truck. Caldwell had been stabbed to death.
Woods is being held on $1 million bail.
Caldwell had recently served Woods with divorce papers and had a restraining order against him.
