In California, and I would guess most other states, many items about right of way in the vehicle code will end with the phrase "until he or she can proceed with reasonable safety." It's one of the basic rules of the road. If you cannot do something safely, then you can't do it. If you can't change lanes safely, for any reason, then you can't change lanes.
Consider another example. You are waiting at an intersection. You have a red light. The light changes to green, but it is obvious to you that one of the cars in the cross traffic that now has the red light is not going to stop. You have a green light, can you go? No, you can't go. Not until the red light runner has passed through. If you can't proceed safely, you can't go.