Your description does not remove the possibility they were “cut off”
She shouldn’t have “expected” them to be going any speed. She should have observed them long enough to determine their speed and then calculated if she had enough time to safely pull out given the distance at the time. Whether she misjudged the speed or the distance doesn’t really mattter. She is negligent in either case and an accident resulted.
Observing somebody closing in in the rear view mirror takes almost no time so it does not allow one to conclude there was adequate time or space for the driver to react safely. You can watch a car 5 feet behind you for a fraction of a second and realize they are moving faster than you are.

