California’s a pretty friendly place for women traveling alone. Most of the major outdoor attractions are incredibly safe, and even many of the urban areas boast pleasant neighborhoods that welcome lone female travelers.
But you’ll need to take some basic precautions and pay attention to your surroundings just as you would in any unfamiliar place. Carry your car keys in your hand when walking out to your car. Don’t sit in your parked car in a lonely parking lot at night; just get in, turn on the engine, and drive away. When you’re walking down a city street, be alert and keep an eye on your surroundings and on anyone who might be following you.
In rural areas, don’t go tromping into unlit wooded areas or out into grassy fields alone at night without a flashlight; many of California’s critters are nocturnal. (Actually, this caution applies to men traveling alone as well. Mountain lions and rattlesnakes don’t tend to discriminate.)
Some neighborhoods in the big cities are best avoided by lone women, especially at night. Besides the obvious—the Tenderloin in San Francisco  and the Compton, Watts, and Inglewood neighborhoods of Los Angeles —some other streets and neighborhoods can turn distinctly hostile after dark.