[m463]

Maybe my wording is confusing, but the idea is not flawed. It is used to set the speed limit, given the road.

If you want a lower speed limit, you can just set it, and if below the 85th percentile speed, you'll get a less safe road.

The proper way to do it would be to adjust the characteristics of the road so that drivers slow down and the speed variance is minimized.

Same for increasing the limit. A wider or smoother or straighter road may work.