Dare I ask what the 85% mark is?

There was a rule at some point that the way to set a speed limit was to measure the speed of cars using that road, and set the limit so that 85% were under and 15% were over. I don't know how much this is actually used any more.

From a safety point of view the proper way to do it is to decide up front what speed limit you want on that road, and then design it so that drivers feel unsafe going over that speed. If you have to have a sign telling drivers to slow down, you designed the road wrong.

	▲	toast0 3 days ago \| parent [-]
		That's the basic standard for a traffic survey, which is sometimes required for enforcement. California has a speed trap law that prevents accepting radar evidence of vehicle speed unless the speed limit was set by a recent traffic survey, on a local road or school zone, or the limit is 65 for cars and 55 for trucks. Maybe some other exceptions. I 100% agree on build the road to result in the behavior you want. Speed limit signs have some effect, but narrow lanes are much more effective.