I'm sorry, but I think you misunderstood the 85th percentile.
When there is no speed limit posted, people drive what they think is safe. Roughly 85% are going to drive safe speeds, or slower than safe speeds. Another 5 to 10% are going to push things a little bit. That top 5% is pushing things more than just a little. So, you watch the highway, to see how fast people are going, then set the speed limit to the 85th.
Most of those 5 to 10% who were pushing things a little bit, will VOLUNTARILY SLOW DOWN to the new speed limit. First, the difference probably isn't much - they don't feel very inconvenienced, if at all. Second, keep in mind that there WAS no speed limit - now there is one. MOST people will make some attempt to obey a sensible speed limit.
That remaining 5%? They knew all along that they were pushing things. With the imposition of a speed limit, they are likely to slow down, and conform to the flow of traffic. If not - then they'll pay the ticket.
In such a case, the flow of traffic will be at or near the posted speed limit. Contrast that with what we see around most cities. 40% or less is actually doing the speed limit. Doing the speed limit actually makes you a hazard, because your car is the ONLY ONE going that slow.
Or, to state the same thing in a different fashion - any time the majority of traffic is going over the speed limit, the posted limit is WRONG.