The break away part; any object over 5 inches tall with in the 'clear zone' of the roadway should be 'crash worthy'; ie breakaway. That doesn't mean somebody is going to hunt you down if it's not; but some of these mail boxes/posts get ridiculous, and could absolutely kill someone in a otherwise minor injury wreck. Over by the beach, giant concrete manatee mail boxes, like 2 feet from the edge of pavement, in a 55 mph speed limit,... I've seen mail boxes on a rail road rail, poured in concrete; mail box on 8" Ductile Iron Pipe, ect. Never actually heard of a specific fatality tied directly to a mail box, but im sure it happens.