yep, people will naturally drive at any speed they determine to be safe for the road they are on. Those signs help make them aware of it.

I can honestly say that 90% of the time, I probably am not sure what the speed limit of a road is that I am on. I naturally slow down in residential areas, and speed up for highways. Generally this is 75mph or so in my area with a speed limit of 65mph. If they raised the speed limit to 85mph, I personally wouldn't really like it and probably wouldn't drive that speed.

I've gotten only one speeding ticket in my life, and I have never been in any accidents of any sort. The speeding ticket was on a 2-lane straight road, in the middle of the day with barely anyone around, so I was more comfortable with going fast and didn't even know I was going that fast.

edit: if you're going to down vote me, at least add to the discussion and prove me wrong.