With Uber and other ride hailing services it became clear that cheap point to point transport replaces trips that are otherwise being made with public transportation like buses, and thereby increasing traffic. There were also more trips in total done because of the convenience than were done before, thus also increasing traffic. It’s the classic Jevons paradox.
Self driving taxis could certainly have the same effect or more if they are cheaper than ride hailing. The increase in usage can easily be greater than the number of private cars it replaces.