I drive a 2.0L Turbo Diesel Ford Focus, and when I renew my lease next year I'm planning to move down in engine size. Why? Mostly because I've identified that the prime reason I have a 2 litre is vanity; I have it because it goes fast and because it's available to me.
So, given the challenge that cars provide for the environment, how about this for an idea?
What if there were only 4 different types of car available in the whole world? Cars would be graded by size, ie. a 2 person car, a 2 + children, a 4 adult and a people carrier. Within those 4 types, each would be identical, no engine choices, no alloy upgrades, no body styling, no nothing. To pull this off, all of the car manufacturers would pool their resources to make the most efficient 4 cars they could, leading to huge technological advances and the resulting vehicles would then be manufactured en mass. Personalisation would be dealt with on the exterior - each car would be sold in white, and then customers could customise the paint job to their personal and no doubt exacting standards.
Car choice would then become a simple size equation, and given the combined might of the automotive industry I'm sure super low CO2 and super high MPG would be assured, along with excellent drive, comfort and ride. The mass production and removal of intercompany competition would hopefully lower prices to boot.
However, there are 2 problems with my plan. 1) Greed - the big manufacturers wouldn't go for the above because they'd have to share their profits with the little guys. 2) Vanity - us consumers will continue to demand the freedom of choice to buy whatever vehicle we want, even if it is a 4.8L Chelsea tractor that does 9MPG and outputs 350g CO2.