As more and more Chinese engineering construction enterprises go overseas, strengthening the comparative analysis and research of domestic and foreign norms and standards is crucial to the smooth implementation of the projects. By analyzing, the control factors of relaxation curve length are determined. The differences and reasons of the length values of relaxation curve in expressway route plane design of Chinese, Tanzanian and American codes are compared and analyzed. Combined with a project case, the calculation and analysis are carried out from five aspects of driver comfort, ultrahigh transition section setting, linear homogeneity, driver operating time and vehicle lateral displacement. The recommended value range of the length of the relaxation curve corresponding to the radius of each flat curve is obtained, which provides the reference for the plane design of expressway routes at home and abroad.