The problem of overlapping between flat curve and convex vertical curve (or concave vertical curve) in the introduction of set recommendation is a common problem in production practice. When the horizontal curve and the convex vertical curve overlap, the sight distance is obviously shortened due to the perspective imaging effect (so-called convex curve effect), while the concave curve obviously causes the extension effect (concave curve effect). Because of this false paragraph function, drivers will still unconsciously drive at high speed under adverse road conditions, which increases the risk of accidents. Professor R Ram of Karlsruhe University in West Germany has done a lot of investigation and research work, which proves that the concave curve section is a dangerous section with frequent accidents, and only through appropriate safety measures can the accidents be reduced. The argument put forward in this paper has great practical value and can be used as a reference for highway designers and traffic management departments in China. For many years, the highway industry thinks that the relationship between plane and elevation must be considered in highway engineering, because the driver does not know all the individual elements that make up the road system. To be exact, the driver does not judge the driving situation according to the ever-changing spatial curve, but the perspective image of the curve he sees during driving. In this regard, Ostro believes that it is really difficult to distinguish expressway perspective images. Experts repeatedly try to estimate the line elements in the plane and elevation from the driver's point of view, and always make mistakes again and again. Especially when the horizontal curve and vertical curve overlap, in most cases, the estimated value is always very different from the actual value, so that it can not be reasonably explained.