Why can cyanide compounds never be loaded in the same vehicle as acids, even in a case where the general segregation table might otherwise only call for a 4-foot separation?

49 CFR 177.848(c)

Hydrogen cyanide is a fast-acting, highly toxic gas, so this rule exists to prevent a leak from turning into a mass-casualty event inside an enclosed trailer or at a crash scene. Because this prohibition is absolute, 'we kept them 6 feet apart' is not a valid defense if the materials could react if their containers failed.