AbstractThis article summarises three former papers [1], [2], [3], the first of which set out for the first time the fundamental principles behind the design of road curvature, and the two later ones described an extensive series of recordings of the motion of cars on curves. From these latter it was concluded that the then current theory proposed for the design of transition curves was fallacious. A restatement is needed because curvature is still being designed on the obsolete theory, at some danger to traffic.