You have just finished unloading a small amount of hazardous liquid from a cargo tank, leaving only trace residue. Can you move the tank truck with the manhole cover and valves still open since the load is nearly empty?

49 CFR 177.834(j); AAMVA CDL Manual Sec 9.5.2 (nhcdm.pdf p.9-13)

Even a small amount of residual liquid can spill or leak out of an open valve or manhole while the truck is moving, creating a hazmat release on the road with no one positioned to stop it. The rule removes any judgment call about how little is left by simply requiring the tank be sealed before it moves, every time.