Warning: Learning doesn't last.
Lessons learned can be slowly forgotten over time. Memories fade. When something is "learned", is it permanently imprinted in our memories? No. We become complacent again. We forget. We may not forget everything but we forget the details, the how and the why. Lessons may be institutionalized through new rules and processes as a result of an accident -- to ensure it doesn't happen again -- but with the passage of time, it's just another rule, soon disassociated from the original incident or accident. As soon as people no longer understand the "why" associated with a rule or process, it can be dismissed as bureaucratic red tape and soon ignored or frequently bypassed.
Remember Chernobyl? Remember Bhopal? Remember the Tenerife double aircraft disaster?
What do you remember about them?
Everyone remembers the Titanic, but what exactly do we remember about it? Do we need to be reminded of the details of why and how it happened on a regular basis?
We pay most attention to the why and how just after an accident happens because everyone is focused on "how could it possibly happen?" and "who is responsible?" What we really need is a process for reminding people of the why and how when they think they least need it, when everything is going well and they start thinking it could never happen to them.
I'm also wondering about other factors:
1) Proximity: What's the relationship between an individual's "proximity" or level of involvement with an accident or related lessons on the one hand, and the declining memory curve? Does first hand "learning" last longer?
2) Intensity: What's the relationship between the intensity of the failure (i.e. human lives lost vs. a failed project that didn't achieve its objectives), the extent to which the causes of failure are investigated, and the speed with which memories of the failure fade and lessons are unlearned.
3) Dynamic nature of Lessons: Lessons need to be "updated" regularly based on most recent history and discoveries. Even if you've learned something based on first hand experience, you still need to "update" that knowledge.
Rules and mandated processes need to remain linked to their original rationale. When someone is told that they need to follow rule x, y, z, they should be able to ask "why" and to get a straight answer other than 1) that's how we've always done it, or 2) that's the rule. If you understand the why and the rationale makes sense, you're much more likely to follow the rule.