So in a hypothetical wherein Robotaxis have acheived full penetration, maybe we'll need 1/3 of the vehicles we use today.

There would likely be a lot less steel in those vehicles as well, since they'd largely be optimized for single commuters, and would need less of the redundancies built into current cars for safety.

http://data.bls.gov/timeseries/LNS11300000

https://i.imgur.com/5YFhBpx.png

As well as the labor force declining in general:

http://www.factcheck.org/2015/03/declining-labor-participati...

"Still, Priebus’ comment, tying the entirety of the drop in the labor force participation rate to “the Obama economy,” ignores some of the demographic and structural forces that have been driving the participation rate down for more than a decade, and that are expected to continue to drive the rate down for decades to come."