I vaguely recall a conclusion that a safety system which is visibly advertised and promoted will in fact not add to safety that much, because operators tend to take more risks or act more sloppily in the belief that the safety system will save them from any mistakes. Whereas a safety system that just is there in the background without any advertising, will have a real positive effect, basically because operators are not aware of it and thus do not rely on its capability to save them from trouble. As examples from the road traffic sector we have ABS brakes in the former category and ESP in the latter.
Makes good sense to me... noting that it is more likely Operators, as PJ2 hints at more than once, have 'assumed' time honoured practices can be safely shelved when using their SOPS combined with such warning systems.. they then become less of a third-man/back-stop and more vital, being directly in the safety chain.
When engineered to be as fault tolerant as other primary systems and not open to inadvertant abuse (CB pulling etc), then modern-day SOPs and these systems can maybe provide the absolute level of safety sought - but to use that combination with legacy 70's & 80's warning systems seems to be putting the cart before the horse.