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Sight Distance Checking for Road Design

Sight Distance Checking for Road Design
Sight distance is an important design consideration for safe roadway design, stipulated in engineering services guidelines and often forming part of the controlling criteria for highway design.

Stopping Sight Distance
For moving traffic, sight distance allows for the calculation of stopping sight distances along the roadway. Stopping sight distances are a function of the roadway design speed, driver perception-reaction times and driver deceleration rates. For minimum stopping sight distances at any point along a horizontal curve, minimum horizontal sightline offsets provide the basis for calculating the acceptable sight distance around the curve, as represented in the diagram opposite.    minimum horizontal sightline offsets
A minimum horizontal sightline offset

The vertical alignment of the road impacts the line of sight such that the stopping distance is increased on downgrades and decreased on upgrades. Road authority design standards provide equations for the the calculation of sight distances that take into account the grade of the road.

Road Intersection Sight Distance
Design guidance for road features such as intersections and rail crossings may also require the civil engineer to ensure adequate visibility is achieved for stationary vehicles. In this case sight distances are calculated on a point to point basis and the line of sight is primarily affected by obstructing terrain features.

Design interventions to improve sight distance
Where inadequate stopping sight distances are achieved, vertical alignment modifications can be made by 'shaving' the top off a road crest, and slopes can be graded with fill material. Horizontal alignment alterations may include increasing bend radius, providing transition bends, removing compound bends or improving the super-elevation. Sight lines can also be cleared by removal of vegetation or by cutting the embankment on the inside of horizontal curves. Where sight distances are too difficult to improve, speed management or restrictions to vehicle movements may be considered along with appropriate advance warning signs.

Sight Distance Checking in Civil Designer Software
Civil Designer allows the engineer to calculate the line-of-sight between objects located at different points of a design model according to the project specific design requirements. The sight distance analysis results take into account the horizontal and vertical alignments of the roadway, the cut embankments as well as the road profile. The sight distance envelope can be displayed in the vertical alignment editor, allowing the engineer to step through the road layout and determine any sight distance failures.

Point to Point Line of Sight
In addition to stopping sight distance checking functionality, the software includes a Point to Point Line of Sight feature. The line of sight between two points can be graphically displayed anywhere on a terrain model. A clear sight line is indicated by a green line between points. As it is extended, it will switch to a red line if and when the line of sight is obstructed. In render view, a tripod with extended sight line provides a visual representation of the line of sight between the specified points.
Sight distance checking in CIVIL DESIGNER software
Sight Distance Checking in CIVIL DESIGNER

Line of sight represented in render view
Render view verification of the sight line at an intersection

Civil Designer can also generate a rendered 3D fly-through along the road at a specified height for design verification purposes. The design verification takes into account the full 3D profile of the road within the road reserve, and marks areas where there is insufficient visibility in the forward and return direction. View a video demonstration of the rendered fly-through for Sight Distance Checking design verification below.

"The upgrade of the Kommetjie Road and Ou Kaapse Weg to create a dual-carriageway along both routes involved significant earthworks to improve sight distance at the intersection." Paul Faria Pr Eng, Urban Infrastructure at HHO Consulting Engineers. Civil Designer's time saving features facilitated efficient delivery of the project's design phases.

Sight Distance Checking for Roadway Design
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