AutoCAD Civil 3D 2012 can calculate values from the geometric definition of a vertical curve, PVI information, as well as object and eye height.
Passing and stopping sight distances are not used for a sag curve because there is no curve peak to obstruct a driver's view of the roadway surface or another object. In daylight, with good visibility, the driver has a full view of the road on a sag curve. At nighttime, however, the driver can see only as far as the distance of the headlight beam. To design a sag curve, you can use headlight sight distance.
You can display these properties in AutoCAD Civil 3D 2012 using the Text Component Editor. In Toolspace, on the the Settings tab, expand Profile Label Styles Curve . Double-click the default Crest and Sag label style. Click the Layout tab. Under Component Name, select an item such as BVC. On the Text Component Editor, click the drop-down arrow under Properties to display the list of available properties. Scroll down to see properties such as Headlight Sight Distance, Passing Sight Distance, and so on.
Passing sight distance is the distance necessary for a driver to see an oncoming vehicle in order to determine if there is enough distance to allow a passing lane. The passing sight distance is not used often in crest curve design because the curve length needed to meet the passing sight criteria is usually prohibitive.
Stopping sight distance, a widely used criterion for determining curve length, is the distance necessary for a driver to see a hazard on the roadway surface. Governmental guidelines determine the sight distance for passing and stopping requirements.
At nighttime, a vehicle's headlight beams define the sight distance. A sag curve intercepts a headlight beam on the roadway surface and limits the length of the beam. Headlight sight distance measures the top of the headlight beam above the pavement and the maximum headlight angle to determine curve length. The height of the driver's eye is also factored.