A passenger wants to carry a liquid poison classified as Division 6.1 in their luggage on a bus. What does the regulation governing hazmat on passenger-carrying vehicles say?

49 CFR 177.870(f)

High-level liquid poisons pose a severe risk if even a small amount spills in an enclosed vehicle full of passengers who have no quick way to evacuate. The prohibition exists because no amount of packaging can fully eliminate that risk in a confined space with limited escape routes.