How much does that tell me about potential software failure modes that don't kick in until a significant scale (speaking of double-digit percentages of all traffic, these test fleets are not even close to that) has been reached? Or about weird, but potentially fatal side effects of incorporating rules put up by regulators into the software that cannot be tested with today's alpha testing fleets because these rules might not even exist yet? Or about how good all these different AI vehicles of different vendors in very different software and hardware revision states interact with each other (think of situations like HFT trading algorithms that run each other into a doomsday spiral, just with vehicles at an intersection twitching around quickly in weird ways, trying to interpret each others actions)? Or about the hackability of future robotic cars (think for example of those slightly modified fake traffic signs)?
Nothing. That's why regardless of how impressively big these test fleets are, there will be a lot more of these unknowns.
Some of them seem like tail risks to me that are unlikely to dominate fatality statistics even if they were to occur and will be quickly patched or recalled if needed. Many of these hypothetical concerns could also affect existing driver assistance systems and aren't unique to autonomous vehicles. Hacking can also happen with human-operated vehicles. Interaction between multiple self-driving ones can also be tested with experimental fleets by concentrated local deployments.
Nothing. That's why regardless of how impressively big these test fleets are, there will be a lot more of these unknowns.