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Junction Visibility (v8i)

Junction Visibility is used to evaluate visibility at an intersection using plan, profile and perspective views simultaneously.  Evaluate either existing sight conditions or proposed design work.  If evaluating a proposed design, then it is recommended to generate a Composite Model of the proposed design into the existing detail.

This analysis is intended for standard intersections not including Roundabouts.

Junction Visibility is located in MX, from the Analysis menu, select Visibility > Intersection Visibility...


Reference Details

The Junction Visibility panel has four tabs grouping the visibility details needed for this analysis:

  1. Model Details
  2. Visibility Details
  3. Objects
  4. Display

Junction Visibility Model Details tab

The Model Details tab is used to specify model details.

The Through Road String is the string that the eye point is set back from (a distance to be specified on the next tab).  The Through Road String also serves as the reference line for the target point.  It is usually either the edge of pavement or the shoulder of the through road.

The Intersection Center Line is the center line of the intersecting road, and it serves as the reference line for the eye point.

Visibility Details tab

The Visibility Details tab is used to specify the sight details, including the required sight distances, offsets, eye and target heights.

There are two Types of Analysis used to determine visibility:

  1. Line of Sight  This analysis uses a single vector from the eye point directly to the target point.  If the vector does not intersect the surface (or any defined objects), then the required sight distance is achieved and the entire vector is displayed in the Visible color (yellow by default).  If the vector intersects the surface (or any defined objects), then the required sight distance is not achieved and non-visible part of the vector is displayed in the Invisible color (red by default).
  2. Shadow  This analysis uses multiple vectors from the eye point to points on the surface which lie between the eye and the target point.  These vectors increase in grade as they progress from the eye point and the target point.  If the grade of any vector is less than the previous visible vector, then that point is deemed to be shadowed from the eye point.  These shadowed points are projected onto the Line of Sight vector and displayed in the Invisible color.

The Through Road Distance is where to specify the required sight distance.  The Give Way Distance is where to specify the set back from the Through Road String to the eye point.   For Eye and Target Point Details, Height values are measured from the model surface to the eye/target point (use 3.5' for both Eye and Target).

The Offsets for the eye and target points are relative to the reference string direction, positive offsets are measured to the right, negative to the left.  This author experienced much fickleness with the Offset controls.

Uncheck the Wrap Along the Channel option, this is a feature geared to European analysis.

Junction Visibility Objects tab

Since the typical string model reflects the surface location of detail, it is often necessary to compliment the analysis model with virtual objects.  Features such as buildings, fences and guard rail are possible obstructions.  The Objects tab is used to create simple, temporary objects to analyze their effect on visibility.

Existing strings can be assigned a Height, Width, and an Offset for any part of the string.  With ingenuity, snow banks can also be simulated.

These objects can be saved in a file for future use.

Junction Visibility Display tab

The Display tab is used to control the window's display and colors.

Select Next to advance to the Junction Visibility viewer.  All the information in the Visibility Details, Objects, and Display tabs are adjustable from the Junction Visibility panel.


The Junction Visibility Viewer

The Junction Visibility Viewer graphically presents visibility sight lines with the use of several interchangeable widows.  Theses windows are Profile, Perspective, and Report.  The MX Display is also synchronized with the viewer for a plan view orientation.

Each window offers a toolbar, many options on the toolbars are similar across all windows.  These options are:

  • Standard zoom controls
  • Vertical Exaggeration
  • Display  This controls the same options found on the Display tab of the Reference Details panel.  Any adjustments made with this option are automatically seen in all Visibility Viewer windows.
  • Visibility Details  This controls the same options found on the Visibility Details tab of the Reference Details panel.  Any adjustments made with this option are automatically seen in all Visibility Viewer windows.
  • Windows  To select a different window type.
  • Minimize/Maximize  To minimize/maximize the window to the Visibility Viewer panel.

There are also controls at the bottom of the Junction Visibility Viewer.  They are:

  • < Back  Returns to the Reference Details panel.
  • Look Left/Look Right  To toggle between the left analysis and the right analysis.  Left and Right is relative to the direction of the Intersection Center Line.
  • Objects...  This controls the same options found on the Objects tab of the Reference 
    Details
    panel.
  • Save  Saves the current settings.
  • Finish  To exit Intersection Visibility.

The Viewer Windows

Profile View window

The Profile window displays the profile of the sight line and the collinear profile of the analysis model.  The top of the widow displays the required and achieved sight distances.  If the required sight distance is not achieved, then the non-visible portion of the sight line will appear in the invisible color (red by default).  When using the Shadow analysis, all the regions shadowed from view will appear in the invisible color.

Perspective View window

The Perspective window shows the current perspective view.  This is the best window to appreciate the influence of Objects.  The Display button controls the display of the triangulation and the clipping of model information beyond the target.  The top of the widow displays the required and achieved sight distances.  If the required sight distance is not achieved, then the target symbol will appear in the invisible color (red by default).

Report window

The Report window presents the associated data.  The Report window toolbar includes buttons to Save Report, Print report, and add an Output Header.

MX Display window

The MX Display shows the sight lines in plan view.  If the required sight distance is not achieved, then the non-visible portion of the sight line will appear in the invisible color (red by default).  When using the Shadow analysis, all regions shadowed from view will appear in the invisible color.


Manipulating the Viewer

With the aid of the MX Display, it is easy to view the effect of adjustments made to the sight line.

  • Use the Visibility Details button to refine the position of eye and target points.  Notice that adjustments made on the Visibility Details panel are automatic to all windows.
  • Evaluate for both Line of Sight and Shadow type analysis.
  • Use the Spinner associated with the Through Road Distance to adjust the sight distance.  Spin between zero and the required distance to ensure that not just the required sight is achieved but all values approaching it.
  • Repeat the above steps for the other direction.

Select Finish to exit Junction Visibility.


Related Pages

MX Add-In Index

MX Wizard Index

MX Macro Index

MicroStation v8 Program Index





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