What specific type of dip in a road is designed so car headlights can shine far enough at night?

The specific type of dip in a road designed so car headlights can shine far enough at night is called a sag vertical curve. A sag vertical curve is a concave, U-shaped section of road that forms a smooth transition between two intersecting grades, typically connecting a downward slope to an upward slope, or a downward slope to a flatter section. The critical design aspect for these curves, especially for night driving, is headlight sight distance. Headlight sight distance refers to the maximum distance ahead on the road surface that a driver can see using their vehicle's low-beam headlights. In a sag curve, the vehicle's headlights project a beam at a slight upward divergence angle from the car. The geometry of the sag vertical curve, including its length and radius, is engineered to ensure that this headlight beam clears the road surface within the dip, illuminating a sufficient distance ahead for the driver to perceive obstacles and react safely. Without proper design, headlights would shine directly into the road surface too close to the vehicle within the curve, severely limiting the driver's effective visibility and stopping sight distance.