As part of understanding a school bus's physical characteristics and controls, why must a driver check and adjust each exterior mirror system, flat, convex, and crossover, before operating the bus?

49 CFR Part 380, Appendix D, Unit D1.1 (Danger Zones and Use of Mirrors) and Unit D1.3 (Vehicle Orientation); AAMVA CDL Manual Sections 10.1.1 through 10.1.6 (pp. 10-1 to 10-3) and Section 11.5.3 (Traffic Monitoring Devices)

A school bus has danger zones extending up to 30 feet from the front bumper, with the first 10 feet being the most dangerous, plus zones to the sides and rear, and blind spots that can extend hundreds of feet behind the bus. No single mirror shows a driver all of that at once. Understanding what each mirror is designed to show, and confirming it's adjusted correctly, is how a driver actually sees the children the bus's size and shape would otherwise hide from view.