That's the basic standard for a traffic survey, which is sometimes required for enforcement. California has a speed trap law that prevents accepting radar evidence of vehicle speed unless the speed limit was set by a recent traffic survey, on a local road or school zone, or the limit is 65 for cars and 55 for trucks. Maybe some other exceptions.

I 100% agree on build the road to result in the behavior you want. Speed limit signs have some effect, but narrow lanes are much more effective.