The crash was reported at 9:20 p.m. on Vasco Road just north of Camino Diablo Road.
Rupe was driving a Geo Tracker north on Vasco Road when she made a U-turn for an unknown reason, CHP Sgt. Diana McDermott said. A red Toyota Camry traveling south at the 55 mph speed limit then broadsided the Tracker.
Rupe was pronounced dead at the scene.
The CHP is still investigating the circumstances of the crash, including road and weather conditions, whether the woman was wearing a seatbelt, and whether drugs or alcohol were involved.
There is a double-yellow line separating north- and southbound traffic on the stretch of Vasco Road where the crash happened.
A Sig-alert was issued 9:54 p.m. and canceled about an hour later, McDermott said.