Check Sight Distance querry

[Anonymous]

Hello guys,

I'm doing Sight Distance check with lets say 100m minimum Sight Distance.

I got the visibility lines created, BUT when I check the lenghts of the lines they are not 100m.

I know that they are 3D polylines - but even if a flat them they are still away from 100m. It is around 1.80 - 1.90m less.

Any ideas?

[chriscowgill7373]

Can you provide an example drawing?  it would be easier to theorize with data to work with.

[Anonymous]

I have a good idea of the problem.  But as @chriscowgill7373 said, could we seee a drawing.  It would help alot

[Anonymous]

I cannot actually share the exact file I'm working on, but I will generate a random one and will share it shortly.

[Anonymous]

I have made a sample drawing that shows:

Feature line and a zero surface.

I have made Check Sight Distance with a 100m distance.

As you can see the distance lines that the command created are not a 100m in length.

And of course, the question is why?

Thanks

[Anonymous]

^^  up

[jeff_rivers]

Sight distance is calculated along the path of travel of the vehicle.  It is the length of the arc segment.  A straight line will not be as long as this arc segment distance. 

[Anonymous]

Isn't this wrong?

Forward visibility check is against the straight line, not the path you are going to make.

[jeff_rivers]

SSD is the distance the vehicle has to travel, because it's the time needed to stop before reaching the object.  

[jeff_rivers · Accepted solution]

The attached figure will help clarify how Stopping Sight Distance is measured.  

Refer to the AASHTO green book, chapter 3, Elements of Design, section 3-2 Stopping Sight Distance.  While the sight line from your eye to an obstacle is a straight line, the path of travel of the vehicle is what's being measured.  If you are traveling at the design or posted speed, you must have enough time to decelerate to a stop after you see an obstacle in the road.  For a straight road, the line-of-sight and the path-of-travel are the same.

However for a curve, the line-of-sight is straight, but the path-of-travel follows the curved road, and so is slightly longer as the attached figure shows (see distance D highlighted in yellow). 

Separate but similar is ISD, Intersection Sight Distance.  This is called the Sight Triangle, and is for the case where a car stopped on the side street must have adequate sight distance to see an approaching car on the major road (and the approaching car must be able to see the car stopped on the side street).  

The core concept is the same as SSD: The approaching driver must have enough time to decide to stop, react, and decelerate to a stop; the stopped driver must have enough time to decide to enter the intersection, react, and accelerate through the intersection.  

[Anonymous]

Jeff,

Thank you for your exhaustive replay. I'm fully aware of the difference between both - the thing is that here in the UK in some guidances they refer to the line of sight as the measured distance.

So was thinking is it worth at all to have the option to chose between both?

Anyway - thanks again.