Didn't read? That 85% _mantra_ is mentioned. Yes, it's an engineering principle...designed for rural roads. Blindly applying that to all other road types is not engineering, that's cargo cult.
I'm not a traffic engineer so I'd love an explanation, because on the first look, it seems absurd to apply this measure to determining speed limits, as a) 85th percentile will keep creeping up with each iteration of such speed limit adjustment, and b) the whole rationale seems to depend on "wisdom of the crowds", i.e. trusting greedy optimization to find the right balance in the system that needs to be optimized globally.
> a) 85th percentile will keep creeping up with each iteration of such speed limit adjustment,
That doesn't actually occur. In 1997, West Virginia raised its speed limits from 55 to 65 mph on limited access 4 lane divided highways. The 85th percentile speed incrased from 62 mph to 66.5 mph. The compliance level with the speed limit went from 15% to nearly 85%. On interstate highways, the speed limit increased from 65 mph to 70 mph. The 85th percentile speed increased from 70 mph to 71.6 mph. The compliance rate went from 50% to about 70%.
In both cases, the compliance rate went up (in one case, substantially). Also, the Martin Parker study [1] also says that lowering and raising speed limits do not substantially affect actual traffic speeds.