What improvisation is required? A traffic light being out is a standard problem with a standard solution. It's just a four-way stop.

In many versions of road rules (I don't know about California), having four vehicles stopped at an intersection without one of the four lanes having priority creates a dining philosophers deadlock, where all four vehicles are giving way to others.

Human drivers can use hand signals to resolve it, but self-driven vehicles may struggle, especially if all four lanes happens to have a self-driven vehicle arrive. Potentially if all vehicles are coordinated by the same company, they can centrally coordinate out-of-band to avoid the deadlock. It becomes even more complex if there are a mix of cars coordinated by different companies.

That only works if everyone else also treats it as a four way stop. Which they don't, unfortunately.