For S < L . For S ? L . Assuming h=0.6 m, headlight height; and a ? = 1?, upward divergence of the light beam, For S < L . For S ? L . Sag Vertical CurvesHeadlight Sight Distance. . . The centrifugal acceleration should not exceed 0.3 m/s2. The minimum length of the sag curve is:where: A = change in grade, V = speed in km/h. This requirement is only about 50% of the headlight sight distance. .