@submariner- Vehicle emissions are a tricky thing to regulate. What has happened with the increase of fuel economy has been a sort of rebound effect. The average consumer does not reduce vehicle miles traveled when they get better fuel economy. They actually increase the number of miles they travel because the opportunity cost per mile has decreased. More fuel-efficient cars also have smaller gas tanks so there is a perceived decrease in costs when filling up at the pump.
Consumers respond best to price. If you look at the recent recession and the two major gas spikes that occurred during that time, you would see that the price spikes drove fuel prices down (increased conservation) more than any increase in fleet efficiency.
The most effective way to regulate emissions or increase fuel economy is to drive consumer demand for those vehicles. This is done by increasing the price through price regulation or gasoline taxation. If people knew that gas would never drop below $4.00 per gallon, you would likely have large growth in the auto industry and a significant increase in fuel efficiency standards. This is just my opinion, but I have studied the issue extensively.