I chose durable glass as a potential material of the warning sign since it is extremely weatherproof and resistant against chemicals. Modern glass variants are well protected against scratching and corrosion. Found roman artifacts made from glass prove the logevity of this material.
Since it is barely machineable, it is almost impossible that the warnings signs might be reused as building material, which is a phenomenon observable in the surroundings of ancient ruins nowadays. Their size and weight prevent them from being moved by humans.