The amount that people go over the speed limit varies depending on the road conditions, what they think is safe, and what the speed limit is.

So, if a road feels "safe" at 60mph to people, and you put the speed limit as 20 the average traffic might be 40mph. If you put the speed limit as 40mph, it might go up to 55mph. If you put the speed limit as 60mph, it might go up to 65 mph.

But that natural tapering is what lets you find a stable value for a specific percentile threshold.