The idea of traffic reduction is twofold. First is the fact that there can be a lot more carpools because you don't need the owner of the car to be inconvenienced to make the carpool work, so there will be an overall reduction of cars on the road.

Second, and more importantly, when you have only autonomous vehicles on the road, you can make assumptions that all drivers are perfectly rational, and then your safety margins can be smaller, allowing closer travel at higher speeds.

I suspect in the future, much like how when we transitioned from horse drawn carriage to motor vehicle, at first they will share roads, but then there will be autonomous only roads where the speeds will be higher.