The emission trading system used to control SO2 emissions is based on offset credit trading. Power plants with emissions per kWh below target are awarded credits (depending on how far below). Power plants with above target emissions have to buy these credits off the low emission generators to offset their higher emissions. The result is that average emissions stay on target and it is up power companies to decide to reduce emissions or buy credits.
A similar system could be used to control the average emissions per mile of new cars. See
http://pragmatusj.blogspot.com/2011/08/using-offset-credit-trading-to-drive.html for more details
