A recently released tiny study from UC Berkeley gave 13 people personal chauffeurs for a week to see how their travel habits changed. They found their car miles going up 85%, but in most cases it was for silly reasons that would not actually happen. Still, miles will go up with robotaxis -- but congestion doesn't have to increase at all.

See my article at Study suggests Robocars increase congestion for details of the flaws in the study and how we can have almost zero congestion if we're willing.