It's all relative though. 85 in a 70 and 55 in a 40 are essentially the "same" speed, b/c they are 15mph above the posted speed limit.
The posted speed limit is generally the 85th percentile speed, with the 100th percentile speed being the actual design speed of the road. The comfortable speed on roads is very closely related to design speed of the roadway and obstructions. As stated above, if you make a road 2 lanes in each direction with 12-ft lanes vs. 11- or 10-ft lanes, and you open up the horizontal and vertical curves for increased visibility, you will notice a comensurate increase in traveling speed.
If you narrow up lanes, the driver speed will decrease comensurate to the lane width. If you put barriers on one side, you will also see a decrease in speed. It's simply an unconscious reaction. Traffic engineers have been studying it for 50+ years.
I do like how they are taking a second study 3 months from now. That will give them a good data spread, and will let them know how fast people are driving once they are "used to" the stripes.