While this usually will be fine, there are definitely issues when something stationary does pop up.
While caravan-ing to Yellowstone with 3 vehicles, all traveling in the center lane of the freeway, we encountered a small car with a passed out passenger in the middle lane. My bro-in-law swerved with some room to spare, immediately behind him I swerved with basically zero room to spare, and my father behind me (luckily for them in a Suburban, but bad for the man in the VW) had no chance- I was immediately looking in my back mirror knowing what I was going to see.
There is a decent likelihood that a machine could have swerved in time, but nearly zero that a typical human could/would have in our typical following patterns.
Humans route around inefficient practices. Just as human driving speeds are typically unaffected by posted traffic speeds, they will optimize for their typical experience over written codes for how they drive.
One example is that there are a number of small towns with two way streets that are parked on but only the width of two cars. That creates bottlenecks where cars can only travel in one direction at a time. This might seem like a disaster and it certainly wouldn't work on a busy city street, but in these locations everyone adjusts and when two cars approach a choke point from opposite directions drivers are really good about being cautious and pulling to the side to allow the other party to pass.
There are probably thousands of these local quirks around the world. Handling all of these situations effectively in a fully self-driving car will take an advanced AGI.
A object would need to be substantial enough to cause an accident and then roll into or fall onto a highway. That’s far from a 1 per million miles of driving situation. Remember, something falling off a truck would also take a while to slow down.
But human drivers hit deer all the time, and it's often unavoidable. An autonomous driver is probably more likely to miss a deer than a human, due to substantially better reflex time.
To me, every brown mailbox with a white reflector could be a deer coming to the road, to infrared, with a larger lens, it should be able to tell a lot better.
this is bad, but not quite as bad as it sounds. most of the time, you only need to stop as fast as the car in front of you. it's pretty uncommon to encounter a stationary object in the middle of the travel lane. in fact, outside of driving on surface streets in the city, I can't remember the last time I had to avoid a stationary object in my lane.
I'm much more worried by how closely people follow the car in front of them, regardless of visibility. many leave barely enough room to react at all.