Yes, because in the common case, a taxi (driverless or otherwise) drives empty at least some of the time, to pick someone up, thus creating congestion, compared to a private vehicle, which doesn't drive empty.
The cheaper and more convenient you make zero-person and single-person automobile transportation, the more people will use it, and the more congestion they will create.
The more expensive and less convenient you make it, the more trips will use non-automotive, or public transportation, both of which produce far less congestion.
I actually agree with everything here, but on the other hand the decision of whether and how to actually build the massive amounts of non-car infrastructure we need to have transport be efficient and accessible without private cars of any kind, is in a whole different place. At least in the US, it's pretty clear that in most areas there is very limited political will, even in the grass roots, for things like "build good high-speed trains" and "dig new billion dollar subways" etc. So I think pragmatically speaking things like robotaxis are going to be the "solutions" that we'll actually get.
(And yes, I agree that that's dumb since the same politicians and voters have no problem indefinitely subsidizing and expanding the massively money-losing infrastructure called Roads at taxpayer expense!)
On the other hand, once a sufficient percentage of cars on the road are autonomous, couldn’t they use cooperative navigation algorithms to improve throughput a whole lot?
There are so many inefficiencies with human drivers—chaotic merging, unnecessary lane changes, blocking of passing lanes, and so on. I could imagine that optimizing all those away would make a huge difference overall.
You could also probably increase speed limits. And fewer accidents should cause a significant reduction in traffic jams.
The cheaper and more convenient you make zero-person and single-person automobile transportation, the more people will use it, and the more congestion they will create.
The more expensive and less convenient you make it, the more trips will use non-automotive, or public transportation, both of which produce far less congestion.