New Hampshire is one of a number of states working on developing a regional cap-and-trade program – although “cap-and-invest” is the new term – for transportation. It would work sort of like RGGI, the Regional Greenhouse Gas Initiative that has helped reduce our emissions related to electricity production (and which has just gotten a big boost because coal-heavy Pennsylvania is joining).
NHPR has a story about how such a system might work (read/listen to it here):
The idea is to set and then incrementally lower a limit on emissions from transportation fuels. Fuel companies – likely importers and wholesalers – would have to buy the right to distribute any gas and diesel in excess of that limit in participating states.
Conceptually it’s very similar to RGGI, although I’m sure a zillion details would be different.