Wednesday, 20 June 2012

Sight Distance at Existing Junction

This post is an update to a previous one on the same topic, this one is hopefully more complete.

We have an existing junction and we need to improve the sight distance.
You can use a number of methods to check whether you are getting the required sight distance along your sightline.

1. You can create a profile along the sightline and check the sightline against the existing ground surface and see where the obstruction occurs:
or 2. use the visibility tool 'Point to Point' to check.
These methods only provide an answer along a single line. We can see in profile the depth of the earthworks that needs to be excavated. It would be useful to be able to show the extent of the obstructed view area in plan and get an idea of the extent of earthworks that would be required to provide visibility. To do this we can use a combination of dummy corridors and surfaces to give us the desired result.

First create a profile along your sightline in plan sampling the existing ground surface. Next create a profile from your eye (height 1.05m) to your target (height 0.26m) - see screen grab below.

Next create an assembly using the LinkOffsetandSlope on the Generic tab of the tool palettes.
Create the assembly wide enough to cover the area of interest:
Create a corridor from your sightline alignment, sightline profile and the above assembly. Create a surface from the corridor using top links.
Next create a volume surface using the Existing ground as the base surface and the corridor surface as the comparison. Set the surface style to 2D Solid Level Banding or similar.
In the surface properties for the volume surface set the analsyis to Elevations, create one range and set the max value for the range to zero.
The result will highlight in plan the area where the drivers view is obstructed by the existing ground surface.
Found this useful? Need to learn more? Check out our Advanced Junction Design course HERE

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