As part of a driver's knowledge of a school bus's physical/operational characteristics, what is the minimum tire tread depth that must be verified during inspection?

AAMVA CDL Manual Section 11.5.2, Axle Components/Steering Axle, Tires, p. 11-6

Tread depth directly affects stopping distance and the risk of hydroplaning or a blowout, risks that matter even more on a vehicle carrying dozens of children. Knowing the exact thresholds, and that a gauge rather than a visual guess is required to check them, is a basic part of understanding how the bus's tires factor into safe operation.