Head light Sight Distance

Sag curves do not present any problems of sight distance during the hours of daylight or on well-lit streets at night. The minimum curvature on sag curves is thus much lower than that on crest curves and the main criterion of sag curves is passenger comfort. Headlight sight distance, therefore, applies to driving at night on an unlit road with particular reference to sag curves. 

The values attached to headlight sight distance are, in fact, those of stopping sight distance. The driver’s eye height of 1.05 meters is replaced by the height of the headlight as the points of departure for the line of sight. The headlight is assumed to be mounted on the vehicle at a point 0.60 meter above the road surface and the cone of light is taken to have a divergence of 1° from the central axis of the vehicle.