The Supreme Court of the United States held in Commonwealth Edison Co. v. Montana, 453 U.S. 609 (1981), that in the absence of federal law to the contrary, states may set ecotaxes as high as they wish without violating the Commerce Clause or the Supremacy Clause of the United States Constitution.
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Spain reduced taxes for cars that produced less CO2 (some of which will be exempted), while the more consuming, like SPV and 4WD saw their taxes increased.
The Liberal Party of Canada under Stéphane Dion placed the environment at the front of its political agenda, proposing an ecotax and tax shift it called the Green Shift.
Because HOT lanes and ETLs are often constructed within the existing road space, they are criticized as being an environmental tax or perk for the rich ("Lexus lanes").