Sight Distance Check Results

This issue came up back in December but I have had an unusually busy January so haven't had time to post about it. All credit to James Curran at Donegal County Council for identifying the solution to the problem...

The sight distance check in Civil 3D allows you to check the sight distance at intervals along a corridor. You can specify the eye height, target height, minimum sight distance required and also the interval at which to check. The results can be output to report format and will give you the following information at each chainage interval:

• the actual sight distance achieved
• the minimum sight distance required
• whether the minimum sight distance was violated
• the location of the obstruction point

Below are the the reports from three checks using the same settings along a corridor but checking for different minimum sight distance. The first check was for 60m and it passes, reporting back that there is no violation and we get 60m sight lines at all locations.

Next we checked for 120m. This time we get a violation at a number of chainages. Looking at chainage 1440m we see that the actual sight distance is reported as 49.268m. This doesn't seem to be correct given the previous results.

Next we checked for 160m and got a different actual sight distance (89.956m) value for the same chainage.Again this doesn't seem to make sense given the other results.

So what is happening here? The issue appears to be what the software calculates as the actual sight distance. Taking a not to scale AutoCAD drawing provides an explanation of the results. First the 60m check.

We can see there is no violation - everything ok here.

Next the 120m check.

Here we can see the sight line in red. The actual sight distance as measured by Civil 3D is from the target point to the intersection of the sight line and the ground profile. However the actual sight distance  would be tangential to the ground profile as below in blue and would give a longer actual sight distance value.

Technically the software is correct in how it is calculating but I think calling it 'actual sight distance' is misleading or at the least there should be a better explanation within the command of how and what it is calculating. Maybe it should also give distance at tangent and call it available sight distance.

With what it is calculating at present for the actual sight distance you would need to run the command for increasing or decreasing increments of minimum sight distance and check the report each time to see when you get a violation to properly identify your actual sight distance.