Having (on average) a single person take up 40 square feet of space on the road for travel is horribly inefficient. You do a few orders of magnitude better with a full train car or bus.
Think about uses like DoorDash. Instead of (in a city) walking to nearby restaurants, we now replace a larger share of those dinners with ones where cars are driving back and forth across the city to pick up the food and deliver it. This will only become more common when the cost drops by not having to pay a human being, and so we can expect a much greater volume of traffic from this one example use.
Other currently-marginal use cases will see equivalent growth due to the similar economics.
Re traffic: with robot-driven cars, congestion pricing would be technically easy. This is an example of not considering possibilities created by the new tech.
Realistically, what breaks efficiencies in the US is city design: Places of work mostly detached from activities and housing, pretty much guaranteeing inefficiency. The robotaxis don't make a difference here: The max number of people looking to travel at once doesn't change, and neither does their destination. This makes the number of vehicles actually on the road not change very much at all: The best you can do is hope for people being comfortable (and not inconvenienced) by carpooling with strangers, not unlike what lyft and uber offer already. In the middle of the day, there's no new demand for robotaxis: They'd be parked somewhere, and every mile they move is a car/mile of congestion they create. So you either park the cars where the jobs are, getting, in practice, the same results as private vehicles, or you park them somewhere else, increasing total congestion. It's not just robotaxis that would park: See what happens in all the train commuter lines in American cities, where most trains just get parked in a yard.
The miracle of cities with top transit (See, for instance, Madrid), is that a relatively high percentage of the network is about equally busy in both directions. Trains might stop in rush hour, but route 6 will be useful in either direction most of the time, with little waste compared to US cities. This is the real weakness of the argument of just adding more transit: What we need to make it work is basically urban rebuilds. IMO still a good idea, bit it's a far slower, and more expensive problem than it might appear.
In fact, today the area around elevated freeways is often much cheaper than the surrounding area for several blocks.
People with lower income benefit from having a decent bus system that doesn’t require them to pay $10K a year on a car.
They make not taking your car with you a bit more feasible, and that’s a start at least.
Could you pick a worse example?