> people will end up driving more because of the lower marginal costs.

I suspect places will have to move to either London-style congestion charges or milage-based road taxes to offset this. With an admittedly big loss of privacy, you could even price at the level of individual roads and put Uber-style surge pricing on the roads themselves.

Another possibility is that the "markets in everything" self-driving companies win and cars become a capital asset that can work itself. No reason to drive your car around uselessly if you can rent it out instead, and in theory this could result in fewer cars. However I'm skeptical.