In Virginia, a tree in the road is the responsibility of whoever is in charge of maintaining the road, either the city or VDOT. I would think things would be similar in California.
If none of the affected people are interested in doing ANY work to get themselves out and ONLY want you to do all the work, it sounds like you have terrible neighbors. The neighborly thing for YOU to do is to take the lead at cutting out a section so that cars can pass through. If you don’t already own a chainsaw, weren’t already planning on getting one, I don’t believe it is your responsibility to shell out for one or shell out for someone to cut it down. Asking around if anyone has one you can borrow, watching a youtube video on how to use it safely, and trying to remove a 10 ft section yourself is about as far as I would go if I were in your situation. If no one comes out to help you, take note and don’t plan to rely on your neighbors when the next mudslide/wild fire/earthquake happens.