The 40% number isn't very informative. The report has multiple notes about it:
ODI analyzed data from crashes of Tesla Model S and Model X vehicles involving airbag deployments that occurred while operating in, or within 15 seconds of transitioning from, Autopilot mode. Some crashes involved impacts from other vehicles striking the Tesla from various directions with little to no warning to the Tesla driver.
ODI analyzed mileage and airbag deployment data supplied by Tesla for all MY 2014 through 2016 Model S and 2016 Model X vehicles equipped with the Autopilot Technology Package, either installed in the vehicle when sold or through an OTA update, to calculate crash rates by miles travelled prior to[21] and after Autopilot installation.[22] Figure 11 shows the rates calculated by ODI for airbag deployment crashes in the subject Tesla vehicles before and after Autosteer installation. The data show that the Tesla vehicles crash rate dropped by almost 40 percent after Autosteer installation.
21 Approximately one-third of the subject vehicles accumulated mileage prior to Autopilot installation.
22 The crash rates are for all miles travelled before and after Autopilot installation and are not limited to actual Autopilot use.
So the actual rates of crashes for Teslas using Autopilot vs Teslas not using Autopilot aren't reported.
Reading it, I think it is essentially a 'before October 2015' vs 'after October 2015' comparison of overall accident rates, given the caveats there. So the -40% here should be a lower bound, since it includes a time period with 0% Autopilot usage (before October 2015) and a time period with an unknown but <100% Autopilot usage (after October 2015). If people after October 2015 used Autopilot for 50% of their mileage, then you'd expect the true effect of Autopilot to be more like -80%.
This strikes me as suspiciously high: what fraction of airbag-worth accidents are supposed to happen on the highways where one can use Autopilot at all? So maybe there's some other trend going on which is driving this decrease.
This accident reduction is driven not by the auto-driving part of the package, but the emergency response part. The autonomous breaking and evasive steering that occurs when an accident is imminent is what is preventing accidents.
I'm not sure that it matters as much as you imply.
Even if the only benefit of Tesla's autosteer is that it makes you more focused in the parts that are actually dangerous, a 40% improvement is still a 40% improvement.
That really looks like a terribly small dataset on the "before autosteer" side. According to footnote 21, we are looking exclusively at the sum of miles Teslas had on the clock when someone paid for the post-factory activation. Due to the growth of Tesla sales, that set is likely biased quite a bit towards younger cars with few miles at time of update availability. Tens of millions of miles might sound impressive for a statistical base, but tens of airbag activations does not.
At least they did not compare "all Teslas" vs "Teslas with Autopilot Technology Package checkbox checked", since that would select for drivers who do a lot of miles in boring long distance driving. That kind of driving happens to be the one where any car has an "airbag mileage" far above average, so it would be just natural that they have a lower crash rate, autosteer or not.
But a part of that effect might still be present in the numbers if customers are less likely to pay for a post-factory update than while ordering the car (which I do suspect). If this holds, then the subset of drivers who activated post factory (with miles already on the clock, so that this part of their car usage appears in the "before autosteer" column) will have, on average, a bigger desire for autosteer (and more naturally safe long distance cruising in their driving patterns) than the average of those who got the car with the option factory-activated (and who thus appear only in the "after autosteer" column).
Edit: oh, and here's the really big one: the "before autosteer" number only includes cars that where upgraded. How is this important? All airbag activations that happened in an event that messed up the car beyond repair cannot appear in the "before update" dataset, because nobody spends thousands of dollars on an autopilot update for a car wreck.
[edit 2]: This should actually make the "before" number much better than the "after" number, so i really wonder what's going on here.
ODI analyzed data from crashes of Tesla Model S and Model X vehicles involving airbag deployments that occurred while operating in, or within 15 seconds of transitioning from, Autopilot mode. Some crashes involved impacts from other vehicles striking the Tesla from various directions with little to no warning to the Tesla driver.
ODI analyzed mileage and airbag deployment data supplied by Tesla for all MY 2014 through 2016 Model S and 2016 Model X vehicles equipped with the Autopilot Technology Package, either installed in the vehicle when sold or through an OTA update, to calculate crash rates by miles travelled prior to[21] and after Autopilot installation.[22] Figure 11 shows the rates calculated by ODI for airbag deployment crashes in the subject Tesla vehicles before and after Autosteer installation. The data show that the Tesla vehicles crash rate dropped by almost 40 percent after Autosteer installation.
21 Approximately one-third of the subject vehicles accumulated mileage prior to Autopilot installation.
22 The crash rates are for all miles travelled before and after Autopilot installation and are not limited to actual Autopilot use.
So the actual rates of crashes for Teslas using Autopilot vs Teslas not using Autopilot aren't reported.