Watch Where You Stand

Off-Tracking of Right Turning Tractor-Trailer

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Steven M. Schorr, P.E., President of DJS Associates ::::
A tractor-trailer operator was executing a right-hand turn at a four-way intersection in a major city. As he was completing his turn and heading straight down the roadway onto which he turned, he was flagged down by a person who advised him that his truck contacted a pedestrian.
The physical evidence indicated that indeed the right side of his trailer did contact the pedestrian and knocked him down whereupon the pedestrian was run over by the right rear trailer tires of the right-turning tractor-trailer.
These dynamics are consistent with the properties of a right (or left) turning tractor-trailer wherein the tractor pulls the trailer. The trailer itself has no steering therefore as a result, in a right-hand turn, the right rear tires of the trailer will always track to the inside of the path of the right front tires of the tractor. This concept is referred to as “off-tracking”. The longer the trailer, the further to the inside (of the front tractor tires) the rear trailer tires will “off-track”.
To compensate for the off-tracking, a right-turning tractor-trailer operator often steers to the left prior to executing the right turn. This is why you see signs on some trucks “THIS VEHICLE MAKES WIDE TURNS”. As such, a pedestrian could be standing on, or near the roadway and not be contacted by the turning tractor, but still be contacted by the trailer and/or trailer tires that are moving to the inside of the path of the tractor.

Off-Tracking of Right Turning Tractor-Trailer
Off-Tracking of Right Turning Tractor-Trailer

 

In this case, there was a pedestrian signal defining when the pedestrian would have been permitted to cross the roadway. The major issue was whether the pedestrian was properly waiting on the sidewalk for traffic to clear, or whether the pedestrian stepped into the roadway prior to receiving the walk signal.
Witness testimony (as it sometimes is) was contradictory. Some people had the pedestrian on the corner and described the trailer moving onto the curb striking the pedestrian. Others stated the pedestrian was in the roadway and started to walk into the roadway after the tractor cleared (while the trailer was still turning).
Physical evidence (blood/tissue) and the point of rest of the pedestrian allowed for a determination as to where the right rear tires of the trailer were at the time of the collision.
Utilizing measurements and specifications of the tractor-trailer along with a physics-based engineering program, the accurate turning path of the tractor-trailer was defined in order to establish whether the vehicle moved, in any manner, up onto the curb/sidewalk prior to, or at the time of the collision. The engineering analysis (completed in 2D and 3D so the view of the tractor-trailer operator could also be evaluated), showed clearly that the tractor-trailer was on the roadway during its entire turn path and that the only way the pedestrian could have been struck was if the pedestrian had moved into the roadway, into the path of the trailer portion of the right-turning tractor-trailer.
The analysis was supplemented with 2D and 3D engineering scale diagrams and an engineering animation demonstrating the dynamics of this unfortunate event.
Steven M. Schorr, PE, President of DJS Associates is a Collision Reconstruction Engineer. He can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.

 

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