If autonomous vehicles werebto ever become cheap and reliable, I fear induced demand could have a much larger effect on traffic volumes than the role it plays in traffic now when vehicles rely on human occupants. As people find more creative use cases for autonomous vehicles, the roads would fill up with ghost cars performing any number of progressively more trivial tasks until traffic returns to equilibrium.

Quite possible.

The constraining factor on SDC commonality in the short-to-medium term will be maintenance. These cars require dedicated service (which is why they'll likely start as replacements for taxis, not privately-owned; people will have to be hot-rodders to want to do all the work necessary to keep the sensors clean and tuned).

There may be a threshold in the future where the sensors are hyper-rugged or self-maintaining, but it's a little ways off. The maintenance burden will keep the ghost cars down in the short- to medium-term.