Per ELDT Unit D1.1 training requirements, why is instruction on "dart-outs" specifically paired with mirror system training?

49 CFR Part 380, Appendix D, Unit D1.1 (mirror-system instruction and dart-out awareness listed as required topics within the same unit)

This pairing reflects a real safety principle: equipment (mirrors) and behavior (student education, driver vigilance) work together, and relying on only one creates a gap that has historically led to preventable injuries and deaths near school buses.