They don't expect people to stay under a limit. They set limits based on what they expect 15% of people to break, actually. At least, that's how it works when they put any thought into it at all. Sometimes they just assign an arbitrary default based on the type of road it is.
Congress designated a national speed limit of 55 mph in the 70s to conserve energy. Now many states are looking to raise it (several have already).
Congress designated a national speed limit of 55 mph in the 70s to conserve energy. Now many states are looking to raise it (several have already).