It's only really 9 9s of reliability if you assume somebody will definitely be killed as soon as you have a 10-second lapse in concentration, which is obviously not true.
In motorway driving you could probably shut your eyes for 10 seconds every 20 seconds and be perfectly fine almost all of the time. Especially at night time, or other quiet times of day.
Yes but what I'm saying is you don't need 9 9s of good driving in order to get 9 9s of accident-free driving. You only have an accident when you're driving badly and are exceptionally unlucky (I suppose the amount of bad luck required decreases as the badness of the driving increases).
In the scenario I mentioned you'd have at best a 50% duty cycle of good driving, but the accident rate would be substantially lower than 50% every 10 seconds.
In motorway driving you could probably shut your eyes for 10 seconds every 20 seconds and be perfectly fine almost all of the time. Especially at night time, or other quiet times of day.