It’s probably best if a driver yields to a flying saucer.
Give up your right-of-way when it will help prevent collisions.
When entering traffic, you must proceed with caution and yield to the traffic already occupying the lanes.
https://www.dmv.ca.gov/portal/handbook/california-driver-handbook/laws-and-rules-of-the-road/
It doesn’t matter if anything is behind you or not: any other road users would also be obligated to give up their right of way (by stopping) if you chose to stop, if doing so would help prevent collisions.