The highway manual manual is based off gut feelings and little else. Why 80%, where did that come from? How is the currently set speed limit impacting the survey? If you go to a speed trap town by the highway, you'd say the speed limit is dead on, as no one speeds since they know its a trap.
Roads should be designed for the speed you want, and not the other way around. Your last paragraph is dead on. Real solutions look like this: https://m.youtube.com/watch?v=bglWCuCMSWc
I work in automation. We had one site in Florida that despite AC pumping all the time, had humidity issues. Electonics seemed to handle it ok, but paper was another story. This site would have to print information packs to get packaged up with the product, and it was a nightmare with the collating and folding automation, it just never wanted to work properly. The printers themselves would jam and have more issues than they should as well.