I thought that was just for porn?

Honestly, the 85 percentile rule, when actually used, is about the same as RNG, but with a bias for higher. Iforget where I saw it, but from what I remember seeing even the 85% rule gets deemed as too resource intensive so a speed limit from a “similar” (for some random definition of similar) road’s speed limit is used.

I feel like it was something in the vein of a Strongtowns or Climate Town video.

Edit: Which now that I think about it, I’m reasonably sure the video also referenced the MUTCD.

It’s bad road design. US roads are nearly all designed to encourage high speed travel by being mostly straight, perfectly smooth (well, until weather happens), and super wide. Then we slap a random-ass speed limit sign down and say “job’s done.” If roads were a bit less wide, even if just painted narrower, not dead fucking straight, and if you want to get fancy use something like how the Dutch use bricks for lower speed road surfaces, the road design alone would encourage lower speed driving.