This standard doesn’t universally apply. There was an infamous S-Curve on Lake Shore Drive near downtown Chicago that had a 15 MPH speed limit ... and it MEANT IT. That really was the engineered speed. Every weekend some car with out of state plates would find itself stuck halfway over a Jersey barrier.
Around little Murphy, NC close to the church my grandparents went had a turn that was just about 90 degrees. I think it had a 5 mph speed limit sign.