If they'd pass a federal law requiring stop lights to be timed with each other when closer than X feet from each other, it would save ten times the fuel that a national speed limit would save. I get 17 mpg in the Lambo at 50 mph, I get 5 mpg when I'm accelerating, and if I can go five miles at 50 versus 5 miles having to stop and start 20 times, I've saved a significant amount of fuel.