UT Researcher’s study used in Freakonomics article in New York Times
Kara Kockelman of the University of Texas at Austin, along with John Bottom and other contributors, prepared a report on the topic for the Transportation Research Board (the gold standard of transportation bodies). It showed that being on a road with a 65 mph limit instead of 55 mph means a 3 percent higher probability of a crash taking place.
Much more significant is the fact that the extra speed makes the crashes that do occur far more deadly. Kockelman et al. estimated that the difference between a crash on a 55 mph limit road and a crash on a 65 mph one means a 24 percent increase in the chances the accident will be fatal. Along with the higher incidence of crashes happening in the first place, a difference in limit between 55 and 65 adds up to a 28 percent increase in the overall fatality count.
In addition to blood, the increased speed limit is costing us treasure. While the difference between 55 mph and 65 may not seem so large, the relationship between speed and fuel economy is highly non-linear due to engine design and the physics of wind resistance. A car that gets 30 mpg at 55 mph gets about 27.5 mpg at 65 mph and 23.1 mpg at 75 mph. Higher speeds thus mean greater fuel costs for motorists and more dependence on foreign oil. This was the reason the national limit was enacted in the first place.
Of course, higher speeds and reduced fuel economy mean more greenhouse gas emissions as well.