MX ROAD Junction Visibility Analysis

Hi Ken,

Yes, your analysis of how the junction visibility tool works is correct - it's a line of sight check between the eye and the target point. So it is possible to achieve visibility at the required Y distance, but not achieve it at a distance of less than this.

Ideally, there would be a check box option to check intermediate Y distance, and the tool would advance along the through road edge string, starting at the intersection with the side road centre line and heading towards the defined Y distance, so that you could find the achieved Y distance. But that's a potential enhancement for a future version.

While I was checking this, I found that it was pretty quick to simply click and hold down the spinner to change the Y distance downwards, and see if there was a failure at an intermediate Y distance.

As an alternative technique, one thought that I had was to do an isopachyte analysis. You could create a model of the visibility splay, by creating a single point string at the eye point, at the eye height, and a second string coincident with the through road edge, but offset vertically by the eye height. This second string would need to exist for the Y distance in both directions. Triangulating these two strings would give you the peach area in your picture, in 3D. You would could then do an isopachyte analysis between this and a triangulation of the road design. If the two intersect - i.e. if there's a cut volume - then you know that you have an issue.

You could automate this procedure using an input file. For example, you could use EDIT to create a point at the intersection of the through road edge and side road centerline strings, then DESIGN to create a circle centred on that point with a radius of the Y distance. Then you could COPY the through road edge inside the circle (which would 'clip' the string to the correct length) then DESIGN the vertical offset for the eye height. Similarly, you could use DESIGN to offset the through road edge by the give way distance, then EDIT to create a point at the intersection of the offset string and the side road centerline, and so on. You could use a COMMAND MACRO to automate this further, so you could set up variables for the string names, Y distance etc., then call the MACRO multiple times from an input file - passing in the different strings each time. Whether this would save you time really depends on how many junctions you need to check and how comfortable you are with these techniques.

Hope this helps,

Jon