I remember reading somewhere that drag coefficient depends on the speed at which it is measured.
And this makes sense - for example, see how airplanes are designed. The higher the speed, the less deviation away from a flat angle is allowed.
Although it's indeed possible to conserve fuel by slowing down, this seems to me to be a quick fix. Better to optimize the highways we have and reduce the time it takes to get from place to place and also design around that rather than plan for congestion.
To this end it would be nice to see Cd/ speed curves from the auto manufacturers. I doubt that the econobubbles are going to be that good above 50mph, whereas something very low and sporty could likely have a best fuel economy at 80 mph.
It's something to think about.
Mighty Mira 
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