[twistedwrench]

I would guess, here in California, tracking your miles driven. Replace the gas tax with per mile tax.
And here’s why:
The law encourages an integrated approach to reducing emissions – changing land use patterns to reduce the need to drive, investing in mass transit and other alternatives to driving, and increasing the cost of driving and parking to encourage the use of these alternatives. But it will be up to regional and local leaders to turn the vision into reality,

http://californiawatch.org/dailyreport/report-state-struggling-reduce-vehicle-emissions-8730

[Hardraade]

And you might see another subset of American cars going to the chopper if they cannot be adapted.