It has been my opinion for years that 'most' speed limits (at least on open highways) are set lower than the most reasonable speed. There is a lot of psychology involved in setting & enforcing speed limits, and if limits were raised, many would assume that they could then exceed the NEW limits by the same %.

My personal preference would be to have the signs indicate the ABSOLUTE maximum, with severe penalties for going faster, but people's heads aren't set that way, and it ain't gonna happen.

(BTW...for the first time in my life, I own a car that will easily accelerate and keep up with any normal traffic flow. I now CAN speed with little effort, and it requires me to make extra effort to be sure I'm not just zooming along at 75 like the others. I have cruise control, and it is great, but of little use in areas of high volume traffic.)