Level 2 requires the driver to choose whether to intervene at all times. This is an unreasonable task for humans.
Level 3 puts the car in charge of when intervention is needed, and even once it wants intervention it still has to maintain safe control for several seconds as part of the system spec.
Level 4 puts the car in charge of when intervention is wanted, but you can refuse to intervene and it has to be able to park itself.
So I will double down on my claim. Until the car requests intervention AND the timer runs out, level 3 and 4 are the same. They require the same abilities out of the car. And that section of time, between wanting intervention and getting intervention, is the hardest part of level 3 driving by far. If you can solve that, you're 90% of the way to level 4.
A level 3 car has to be able to handle emergencies several seconds long, and turning it into level 4 is mostly adding the ability to park on the shoulder after you get out of the initial emergency.
The gap between 4 and 5 is a bunch bigger. A level 4 car can refuse to drive based on weather, or location, or type of road, or presence of construction, or basically anything it finds mildly confusing. 5 can't.
I edited a bit for clarity, but also I'll append a thought experiment as an extra edit:
A level 3 car with an hours-long driver intervention timer is basically identical to a level 4 car.
If you have a 0 second intervention timer, you're barely better than a level 2 car.
How long does the timer have to be before developing your level 3 system is almost as difficult as a level 4 system?
I don't think it's very long.
I still stand by level 3 != level 4 in terms of real world liability.
Level 3 allows too much wiggle room and sloppiness to be able to legally shift liability away from the driver. At that point you're playing that "intervention period" length. Manufacturers claiming Level 3 will want to lower it as much as possible, regulators raise it. To me, Level 3 simply shouldn't exist.
Only at Level 4 is the expectation, without a doubt, the machine is in control. A person in the driver seat is optional because the steering wheel and pedals are as well. When people bought "Full Self Driving" they seriously believe "when can I go to sleep?" ability is where it belongs, which always put the expectation at Level 4.
It looked like the Mercedes system is 10 seconds which seems like plenty to me.
And while it would be nice to sleep I'll be pretty happy just looking away from the road.
This isn't correct. Level 4 doesn't require driver intervention[0]. Hence, why I'm arguing "Full Self Driving" starts here, level 4.
So now if you're explicitly not required to "be present" then the system should be liable or at least the "driver" isn't to blame directly.
It's actually level 5 is the same as level 4 but add heavy rain, snow, ice, name-your-adverse-condition.
[0] https://www.sae.org/blog/sae-j3016-update