You're operating on a completely archaic understanding of AI. Modern systems can and do adapt to situations on-the-fly. What's more, they can adapt on aggregate - the experiences gathered by a single car can be shared to all cars. Very quickly, every car in the fleet will have billions of hours of cumulative driving experience.
The overwhelming majority of motor vehicle accidents aren't weird and unpredictable edge cases. They're tragic but mundane events that result from a handful of root causes - inattention, excess speed, poor judgement and unnecessary risk-taking. "Driver/rider failed to look properly" is the key contributory factor in nearly half of road traffic accidents. Computers utterly dominate humans in this respect. For every accident caused by some bizarre and unpredictable set of circumstances, there are thousands caused by someone doing something obviously stupid.
A computer can be programmed to be ultra-cautious in difficult situations. A computer can maintain 100% vigilance 100% of the time. Humans can't. Self-driving cars will undoubtedly fail in new and unexpected ways, but it's abundantly clear that they'll fail much less often than humans.
That's sort of my point. Accidents are rarely caused by edge cases, but by predictable human failures. All the information needed to avoid the typical accident was plainly available, but the driver just didn't see it, process it or react to it appropriately. Driving is generally very predictable, but human attention and perception is hugely fallible. An AI that gets the basics consistently right but occasionally freaks out in an unpredictable situation would be a huge improvement over human drivers.
In the same way, if the world is modelled accurately and succintly, can a car predict the various scenarios and adapt, even if the current sitation was not seen before (extremely high speeds or unusual number of cars, etc.)
From a human's perspective it's a stupid mistake. It seems stupid because computers think differently than people do. On the other hand a computer would probably think a human were stupid if it saw a human make a mistake when calculating the logarithms of every number between one and a million.
It's an improvement as long as those are few percent of the unsurprising and human ways human drivers fail.
The worry is this, the cars will not fail like a human will. It may even be dangerous The car may not even be able to move in certain situations (like another car blocking them in front either purposefully or accidentally). Or it may not be able to even see certain conditions a human could.