Originally Posted by Francis Buxton
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.
85 in 75 is 10mph over. The point I am making is that drivers get used to speed independent of road type. I live on a freeway access road and drivers exit at 75mph and even though the access road is narrower with not shoulder and 40mph posted, drivers typically go 65mph on the access road. They are numb to the speed they are going. 40mph after driving at 75mph feels like standing still, even on a narrow side street.
85% is NOT 85% of the 100% design speed. It is the 85th precentile measured speed on a road after it is build. Settting SLs has become less and less about designed speed and more about the speed most everyone is driving - independent of the varying driving enviroment (x-walks, cyclists, range of driving abilities)
Al