A study from Purdue University found that the majority of drivers say that it is acceptable to exceed the speed limits set on any given road.
Civil engineering and economics professor Fred Mannering says that this attitude stems from the Emergency Conservation Act of 1974 which established new standards for setting the limits on roads. "Now, there are some roads where the speed limit should be posted as 45 but they end up getting posted at 35 because they expect people to go faster," he said.
His study of 988 drivers showed that 21 percent of them think that it's perfectly safe to exceed the posted limit by 5 mph, 43 percent said that it was acceptable to exceed the limit by 10 mph, and 36 percent said there was no harm in exceeding the limit by 20 mph.
Mannering explains that people routinely drive the speed they feel is safe to do so. The posted limits take things like pedestrian safety, fuel conservation, and area concern into consideration; not just the safest possible speed to drive at.