How Do You Calculate Sight Distance?

What is Sight Distance?

Sight distance is the amount of viewable space available ahead of a vehicle to allow a driver to see and react to an unexpected object in the roadway. Sight distance is a key safety feature that should be included in all road design.

The calculation of sight distance requires knowledge of the driver’s eye height and the height of an obstruction to be seen by the driver. In addition, a driver’s line of sight should not be restricted by obstacles at an intersection or along a roadway.

3.7.2 Stopping Sight Distance

Stopping sight distance (SSD) is the distance needed to allow a driver to detect an object or condition in the roadway that necessitates a complete stop, recognize that the object or condition is present, and safely apply the brakes to avoid a collision. The deceleration threshold is based on studies that have documented the average rate at which most drivers slow to a comfortable stop, which is 11.2 ft/s2 [3.4 m/s2].

Decision Sight Distance

Decision sight distance (DSD) adds a dimension of time to the stopping sight distance to permit a driver to detect an unexpected information source or condition, recognize the source, select an appropriate speed and path, and initiate and complete the maneuver safely and efficiently. DSDD values are substantially greater than stopping sight distance, and are recommended when complex decisions or difficult-to-perceive information sources need to be detected and addressed.

4.2.4 Intersection Sight Distance

Insufficient sight distance is a contributing factor to many intersection traffic crashes. In addition to ensuring that the driver of an approaching or departing vehicle has an unobstructed view, the design of intersections should include sufficient lengths along the roadway to allow for the maneuver of another design vehicle.

5.2.6 Sight Triangles

Sight triangles are areas at the corners of roadway intersections where views of approaching vehicles should not be obstructed. These areas are identified on final subdivision plats and commercial site plans, and are required to remain free of all obstructions that would interfere with a driver’s visibility.

6.2.4 Horizontal Curve Sight Distance

When designing horizontal curves on a road, it is important to consider the obstructions that may impede the driver’s view of the surrounding landscape. These include barriers such as the median barrier on open highways or walls on tunnels.

7.2.6 Vertical Curve Sight Distance

When driving through or around curves, a driver’s sightline is often limited by side factors such as mountain slopes and trees and buildings in horizontal curves, or by the dome of the arc in vertical curves. In order to minimize the effects of these factors on drivers’ sightlines, it is important that designers use a minimum length curve in their designs.

Sight distance is also an important consideration in the design of driveways and roadways, particularly when accessing parcels that are skewed. When parcel circumstances do not allow for this, the driveway or roadway should be designed to have its crest angle at a 90-degree perpendicular to the road it is accessing, allowing a balanced view of approaching traffic in both directions.