The crash happened on Highway 198 at Mooney Boulevard just after 3:30 Tuesday afternoon.
Officers say the woman's Toyota Corolla slammed into the back of a Caltrans truck, which was parked on the shoulder.
She died at the scene.
Part of the highway was shut down as crews worked to clear the roadway.
Officers say they're not sure what caused the woman to crash.
The driver of the Caltrans vehicle was uninjured.
It's unknown if alcohol or drugs were a factor in the crash.