Also, it turns out that driving slower reduces emissions.
From Wired: "The faster you go, the more fuel you burn because wind resistance increases exponentially. Lowering your speed by 5 mph when traveling at 35 to 45 mph will boost fuel economy as much as 10 percent (.pdf), according to a report the General Accounting Office prepared in 2008."
Treehugger takes the Wired article a step further, presenting six reasons why slowing down is beneficial. For one:
"It could save a lot of fuel. Some estimates indicate up to 5%; In 1983, by which time many people were ignoring it, it saved 2.5 billion gallons of gasoline and diesel, or 2.2% of the total fuel used. Oil was a horrible $ 20 per barrel. Wired notes that "The Department of Energy noted in 2008 that lowering the national speed limit to 55 mph would save 175,000 to 275,000 barrels of oil daily."