> mandating fuel efficiency

Despite your other excellent points, this one is "typical American", even though you might not be American. I once went to a conference from Jancovici (He did an excellent job at touring France to explain what the global warming was, its origins, and its solutions), here's an example of his 1.30hrs session:

- 1st slide: The awesome downwards curve of car consumption. Individual consumption was divided by, like, 10 in 40 years. Great!

- 2nd slide: The shameful exponential curve of total car petrol consumption. The he stops and asks people what they think. Of course cheaper cars means more cars on the road and any improvement leads to more consumption! When you're in the audience you're compelled to reckon that no fuel efficiency whatsoever will solve anything related to car transportation.

Here's the rest of the conference, in case you're interested:

- 3rd slide: Basic physics calculations about moving 1t of iron through the streets. Then on and on with half the city space spent in parkings, 3/4 of the city space designated to car transportation, while only 20% inhabitants move around with a car and the rest 80% move around with public transportation. And the accident rate.

So basically the individual transportation is the weapon with which we commit suicide. It doesn't matter how much, since it's an order of magnitude more than any other way of life.

- 4th slide: Solution is mass transit. High density. Shield a question about "mass transit pollutes too". Shield another with "But people want green lawn". Shield the most important one about "Big cities pollute the most", but let's come to that later.

- 5th slide: Microeconomy crash course about the offer curve, the demand curve, the point where they meet. And how tax alters them. Take the example of how tax on wages lower both the demand and the offer of employment, and create massive unemployment in Europe.

- 6th slide: Externalities of the carbon market that should have been included in the cost of the cars. The subventions like you say, the cancers you mentionned, the accidents: Excellent points. Don't forget the road police is currently funded by a flat tax on all citizen, preventing renewables from progressing. So is the DoD, which costs trilions to attack so many countries for petrol. Plus the cost of terrorism, mainly due to attacking countries for petrol. All that should be included in the cost of car per km.

- 7th slide: "If we had a carbon tax", for every liter of gas/petrol/fuel, it would lower the demand for petrol and the offer. We could alleviate taxes on wages, because employment is a complementary alternative to fossils. And we want employment while we don't want fossil energy, and we want externalities to be paid by fossil energy.

Tax on petrol is the tax which is the most "even" per pollution unit, which also means the least costly to implement. Petrol taxes are higher in Europe than in US, which means we're architcturing our cities differently, and that takes dozen years, which means our cities are more ready in the face of fossils-independence. And that's why you're so focussed on "car efficiency" while Europe is a little more focussed on getting rid of the car altogether. (although I reckon we're both far from doing enough)

- And a few more slides about how electricity is better than fossils (hint: Yes electricity is often made from coal, but since it's centralized they can lead to much better ways of designing an ecological procurement); how cities with no cars would be nicer, more economically efficient, with more "good" mixity, more fair to the Third World (read: USA's pretense of WMDs in Irak), and how advance on renewables can be a strategic commercial/geopolitic advance in the 3rd millenium. How a blind approach to it puts the 2050-world through a series of riots and Arab Springs.

- Last slide: While we're at it, let's mention that hydroelectric dams are ecologic up until 10km3, and harmful beyond that, because there's always someone in those conference who's skeptic "because dams hurt the nature".

Conclusion: Drop the ideas of regulating the market with pollution limits, consumption per km, laws about architecture. Only an expensive petrol will trigger the right and most efficient changes, and they all revolve around city architecture and lower footprint. Also drop the idea that a rise in the cost of petrol will lower the demand. Even a $100 barrel doesn't accurately picture the amount of externalities. Even at $100 a barrel, you need to add taxes to compensate the state for the deaths, accidents, cancers, wars and social inequalities created by fossil energy, so that other forms of energy don't bear this cost.