Evaluating Sight Distance Issues in a Three-Dimensional World

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by Steven M. Schorr, P.E. ::::

Case Synopsis: A northbound vehicle stops at the edge of a typical four-legged, stop-sign controlled intersection. The operator looks left, looks right, and then left again and proceeds to move forward into the intersection. While crossing the intersection the vehicle is struck on the left (driver’s) side by an eastbound vehicle. The issue: was the appropriate sight distance available to both vehicle operators and did it meet the applicable engineering guidelines?

Expert Analysis: The collision occurred during mid-summer when the foliage was full.  Prior to any tree trimming and any other changes, a high-definition-survey (HDS) laser scan was completed at the intersection.  The HDS laser scan documented (accurate to within 6 mm) the location of the foliage and all other objects in, and around the intersection.  The HDS laser scan produced a three- dimensional "point-cloud".  The three-dimensional "point-cloud" model created a three-dimensional "world" allowing the engineer to demonstrate, from any location in that three-dimensional "world", the view of a vehicle operator or pedestrian.  Utilizing this three-dimensional "world", the intersection geometry and sight distance were compared to applicable Department of Transportation (DOT) and American Association for State Highway and Transportation Officials (AASHTO) guidelines.  Police photographs taken during the scene investigation were incorporated into the three-dimensional model through the use of computer reverse protection photogrammetry (CRPP).  This allowed the jury to view, within the three-dimensional "world", the precise view depicted in the police photographs.

Result: The High-Definition Survey (HDS) laser scan provided a level of accuracy, completeness and flexibility not presently available by any other data collection method.

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