The long term goal is to tax you directly on how much you drive.
I agree, Redgolum. If you look at this Para 49, 563:
http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&rgn=div5&view=text&node=49:6.1.2.3.31&idno=49
The tables in that paragraph include everything but testing for your blood type. They'll be able to tax more than your miles. They'll be able to tax your hours and minutes, your locations, probably even the air you use.