> If your standard for saying that event x caused deaths is that we have a statistically significant direct correlation between event x and excess deaths that year

It isn't.

> Hell, even something like mass shootings probably aren't enough deaths to show up directly in death numbers for the year.

Yes, there's (probably) no statistically significant link between mass shootings and excess deaths (maybe with the exception of school shootings and excess deaths in the population of school children). But you can directly link 'a shooting' and 'a death', so you don't need to look at statistics to work out if it's true that shootings cause deaths. Maybe if mass shootings started to show up in excess deaths numbers, we'd see a different approach to gun ownership, but that's a separate discussion. You can't do the same with 'closing airspace' and 'deaths on the road'.

When you're looking at a big event like this, there's a lot of other things that can happen. People could be too scared to take trips they might otherwise have taken (meaning a reduction in mileage as folks are not driving to the airport), or the opposite, 9/11 might have inspired people to visit family that they otherwise might not have visited (meaning in increase in mileage). Between 2000 and 2003, the price of gas went down, which might have encouraged people to drive more in general, or to choose to drive rather than fly for financial reasons (although if you wanted to mark that down as 'caused by 9/11' that's probably an argument you could win). You can throw ideas out all day long. The way we validate which ideas have legs is by looking at statistical significance.

Here's some more numbers for you, in 2000, there were 692 billion revenue passenger miles flown in the US. In 2002, it was 642 billion. So we can roughly say that there were 50 billion fewer miles flown in 2002. But the actual number of miles driven in 2002 was 100 billion higher than in 2000 (and note, this is vehicle miles, not passenger miles, whereas the airline numbers are counting passengers). So clearly something else is at play, you can't (only) attribute the increase in driving to people driving instead of flying.

> If you actually think the causal chain breaks down - that 9/11 didn't actually lead to fewer people flying, or that actually didn't lead to more people driving, or that extra driving didn't actually lead to more deaths - then that's worth discussing

I believe that the causal chain does exist but it's weakened at every step. Yes, I think 9/11 led to fewer people flying, but I think that only a proportion of those journeys were substituted for driving, and some smallish percentage of that is further offset by fewer journeys to and from the airport. I think that the extra driving probably did lead to a small number of additional deaths, but again this question of 'is one additional death enough for you to think I'm wrong' comes back.

If I throw aside all scientific ways of looking at it, my belief is that in terms of direct causation, probably, in the 12 months following 9/11, somewhere between 50 and 500 people died on the roads who would not have died on the roads if 9/11 had not happened. But a lot of those were travelling when the airspace was not closed.

If we look at the number of people who died because they made a trip by car that they would have otherwise made by plane but couldn't because US airspace was closed (i.e. on 9/11 itself and during the ground stop on the 12th), you're looking at what I believe to be a very, very small number of people, maybe even zero.