You might think that speed limits are set for safety. You would be wrong.
Across the US probably the most common infraction issued to drivers is for driving faster than the posted speed limit. Most people assume that speed limits control traffic and keep us safe...and, to be fair, for the most part this is correct - it would be dangerous to allow drivers to travel at 75 mph through neighborhoods and school zones. You would think that there was some safety science behind the posted speed limits; some science that concludes driving faster than X speed on Y road increases the likelihood of accidents. Surprisingly, there isn't much and the science that is available does not determine the speed limit set.
Under the Uniform Traffic Code, the speed limits should be set according to how fast traffic actually moves along a particular road. According to the Code, the speed limit should be set at a speed so that 85% of the traffic moves slower than the limit and 15% moves faster. That's right- even if speed limits were set according to The Uniform Traffic Code, it anticipates that 15% of us will be breaking the law.
What actually happens is this: The speed limits are set by the controlling jurisdiction according to how safe people feel driving at a particular speed on a particular road. The science tells us that when using this criteria, in most cases, the speed is set even below the recommended speed using the Uniform Traffic Code criteria; meaning that the speed limit would be set anticipating that even more than 15% of drivers on that road will be breaking the law. Further, there can be no doubt that revenue is a consideration when setting the speed limit. I mean talk about a conflict of interest - the jurisdiction setting the speed limit, planning for some percentage of the population to violate the speed limit set, is also the body that is paid fines for citations issued when the speed limit is violated - they have every reason to set speed limits that anticipate a larger percentage of the population driving in violation.
In the end, profit due to fines is a large factor in the setting of every speed limit. Safety of drivers is only tangentially important in setting speed limits and is based upon average speed traveled and the populations comfort driving at a particular speed, not hard facts relating to accidents or injuries.
For more, here is an article from the Five Thirty-Eight blog.