This isn't really true. Road design is more complicated than people think. There's actually a table (I think in the green book) that gives you what are essentia

This isn't really true. Road design is more complicated than people think. There's actually a table (I think in the green book) that gives you what are essentially coefficients for drivers "subconscious" speed, given certain design features. Meaning that reducing lane width, shoulder width, or even sign size will cause a driver to "naturally" drive slower. Setting the speed limit outside of this natural range happens all the time, usually in good faith, but results in speed traps. So maybe not "physically" difficult, but it certainly takes a mental effort.