This is why I prefer thinking about root solutions rather than root causes. The answer to the question “how can we make sure something like this cannot happen again?” (for a reasonably wide definition of this). The nice thing is that there are usually many right answers, all of which can be implemented, while when looking for root causes there may not actually be one.

The root cause was the deregulation in the 90s and the removal of the Glass-Steagall act.

Complex systems are also simple systems when viewed as a black box from the out side. A lion always eats a gazelle given a chance and a bank always explodes if not regulated to within an inch of its life.

That people like to pretend internal complexity matches external complexity is a very odd mental quirk. It is false in both directions of implication. Conway's game of life is as simple a game as you can get yet it has the most complex behavior possible.

> But what about a case where someone 3 or 4 car lengths ahead makes a quick lane change and an accident occurs behind them?

You'd have to be going extremely slowly for 3 or 4 car lengths to be a safe following distance. On a typical 60-70mph freeway you should have a gap of at least 15-20 car lengths, and then accidents like that will happen only when other factors are at play (and if those factors are predictable like water on the road then your distance/speed should be adjusted accordingly).

While I think that example was bad, your point about there existing accidents without single points of blame is still valid.

> A quick example: in many car accidents, you can easily point to the person who caused the accident

This is a bad example: traffic is a complex sistem with a century of ruleset evolution specifically intended to isolate personal responsibility and provide a simple interface for the users, that, when correctly used, guarantees a collision free ride for all participants.

The systemic failures of trafic are more related to the fallible nature of its actors. The safety guarantees work only when humans demonstrate almost super-human regard to the safety of others, are never inattentive, tired, in a hurry or influenced by substances or medical conditions etc.

We try to align personal incentives to systemic goals with hefty punishments, but there is a diminishing return on that, at some point you have to consider humans unreliable and design your system to be fault-tolerant. Indeed, most modern trafic systems are doing this today with things like impact absorbing railings, speed bumps, wide shoulders and curves etc.

I wrote another comment, before reading the article that says it better than me:

Catastrophe requires multiple failures – single point failures are not enough.

> During and after the economic collapse of 2007-2009 I wondered who was at fault; who to blame. I kept waiting for a clear answer to "THE root cause".

Well, there can be a difference between "who is at fault" and "THE root cause". Quite a large difference, potentially.

In the case of '07-'09, maybe nobody was at fault (seems plausible) but there was a very neat root cause. The government handed out a lot of money to people who shouldn't have gotten it, the people responsible were largely protected from bankruptcy and the system forced to reform largely as it was. The people who took excessive risk earned an excessive reward - they should have all gone bankrupt. The financial system should actually have changed, and people who made productive investments and didn't take risk should have become ascendant. Instead we have the same old crowd playing the same old game.

Disabling the major feedback mechanism of capitalism is about as root-causal as can be gotten. Nobody in particular chose to disable it though, it was a consensus decision among the powerful.

> During and after the economic collapse of 2007-2009 I wondered who was at fault; who to blame

Sometimes it is possible to at least narrow things down to an underlying inherent instability. In the case of your example, a huge underlying cause is an economic system based on debt (backed by interest and usurious transactions). It's for a reason that usury/interest is banned in Islam, Christianity, and Judaism for example. It's a parasitic practice that makes the economy fundamentally unstable. This includes dangerous practices such as selling debt for debt (again part of the same crisis), and things like stock shorting (which, interestingly enough was also banned during the crisis, at least for some critical company stocks).

There is a relatively simple root cause of the financial crisis, it's called moral hazard:

https://en.m.wikipedia.org/wiki/Greenspan_put

Now, how exactly to solve the problem is a complex question, so I suppose in that respect it's hard to think productively about it.