In the context of school bus danger-zone training, what is a "dart-out"?

49 CFR Part 380, Appendix D, Unit D1.1 (requires training providers to teach the dangers of "dart-outs"; the definition given here reflects standard pupil-transportation-safety training terminology, not a verbatim CFR definition)

Most school-bus-related child fatalities happen in the danger zone immediately around the bus, often from a dart-out the driver never saw coming — this is precisely why ELDT curriculum requires explicit training on recognizing and preventing it, not just on mirror mechanics alone.