Before loading, unloading, or moving a tank vehicle, which valves must you confirm are in the correct position?

AAMVA CDL Manual §8.1.1, p.8-8-1

A valve left open or in the wrong position can let cargo escape the moment the vehicle moves, even if the tank shell itself has no leak. Checking valve position every time is what actually prevents a spill during transit, not just at the loading dock.