There's a correlation between the speed limit and the design speed, but there's also a lot of mismatch and the bias usually goes in a specific direction.

A few mph over the speed limit in dry weather is rarely outside the design.

And I think I have a reasonable amount of nuance. If you don't have nuance you get "the sign can't be wrong" or "always go slower". The former is objectively not true, and I've never seen anyone seriously advocate for the latter.