You try to do a good thing, but someone always has to come around and say there's a down side. In this case, the good thing is AB 32, also known as California's greenhouse gas emission reduction law, and the potential down side is one that could generate a lot of bad publicity.
The United States government is in the early stages of crafting the nation's first carbon cap-and-trade law. The House Committee on Energy and Commerce released a white paper detailing the scope of the carbon law and one thing is already very clear: drivers won't be regulated directly because there are just too many of us! You can read the entire 22 page white page pdf here. Transportation is discussed on page 13 of the white paper. Here is exactly how the white paper explains why regulating ind