Collision Reconstruction – Time Distance


Expertise Includes:

    • Vehicle Collision Reconstruction
    • Crash Data Retrieval
    • Forensic Mapping Technology
    • Motorcycle Collision Reconstruction

A common car crash is when one vehicle makes a turn or pulls out in front of another vehicle. Normally, without the accident, the vehicles only cross paths for milliseconds. When the collision occurs it’s no doubt because both vehicles try to occupy the same space at the same moment. The question is often “who is at fault?”. Law enforcement often has the stance that a vehicle cannot pull out or turn until it is safe to do so; hence they  show the maneuvering vehicle, let’s call it Vehicle M, is at fault in the report. Vehicle M typically has the worse injuries. There is truth to that perspective; suicide excluded, why would Vehicle M knowingly make the maneuver without thinking the approaching vehicle, let’s call this Vehicle A, was traveling at the speed limit?  The driver of Vehicle M has a reasonable expectation that Vehicle A is traveling at or below the speed limit.

When it comes to going 10 mph above the posted speed limit,  no one seems shocked by the idea. It has become commonplace and is difficult to enforce.  The tolerance applied by law enforcement, which varies between officers, is unwritten to allow for speedometer error, brief inattention, and changes in the roadway grade. It is next to impossible to maintain a consistent 55 mph on an upstate South Carolina roadway with the hills and curves. Even cruise controls functioning on a vacuum system fluctuate a few miles per hour.  But what if the excess speed Vehicle A was traveling made a difference?

The way to know what could have happened is to do a time distance study of the crash. This will show if Vehicle A was driving too fast or if Vehicle M did not have enough time to make their maneuver. Any crash investigation starts at the end of the crash and works backward. This gives an understanding of what happened leading up to impact.  A time distance study adds in a “what if?” to see what would happen if you change a variable within the scenario and then ‘move’ everything forward again.  Understanding the impact location, the investigator must have an approach speed for Vehicle A. If Vehicle A was traveling over the posted speed limit or driving faster than reasonable for traffic for conditions, there is justification for the research.  If it’s a bright sunny day in a rural part of the state, and Vehicle A was 5 mph under the speed limit then it’s probably not something worth putting the time into, unless there are other defining circumstances. In addition to Vehicle A’s information, knowledge of the Vehicle M’s actions is required:  Was there a stop or yield intersection?; Was Vehicle M speeding up or slowing down?  These are just a couple of considerations that can affect the study, they should be explainable or arguable.

Event Data Recorder

Calculating speed ranges gives a starting point for analysis. Physical evidence on the roadway or from the vehicles assists in this task. In most modern vehicles, the Event Data Recorder (EDR) will have a lot of the details for understanding what the drivers of the crashed vehicles did seconds before and up to the point of the crash.  EDRs capture the speed, accelerator position, braking, and sometimes steering to show what the drivers were attempting to do. With this information, it can be calculated where the vehicles were in both time and distance when the collision sequence began.

Knowing where the collision began, the elements that are of concern can be altered and the collision reanalyzed to see what difference could have been made and how it affects the crash. This analysis could also show that Vehicle M may not have seen Vehicle A if there is a curve, hill, or other vision obstruction that limited the sight distance for the drivers and should have been a consideration for DOT when setting the speed limit in the area. Other cautionary signs in the area warning of curves or intersections will need to be documented.  A time distance study is just another way to locate the truth of what happened in the crash.

Aaron (Al) Duncan II, ACTAR, is a senior vehicle reconstructionist with Warren. Prior to joining Warren, he worked for 23 years as a South Carolina Highway Patrol Trooper to include 10 years as a Multi-Disciplinary Accident Investigation Team (M.A.I.T.) member. Al has investigated in excess of 1000 vehicle accidents and incidents, as a trooper. Al has testified multiple times in state courts and has been a court qualified as an expert in accident investigation and collision reconstruction. Al’s work expertise focuses on investigating and reconstructing vehicle collisions involving single and multi-vehicles, animals, pedestrians, motorcycles, heavy trucks and commercial vehicles. He is also a skilled user of forensic mapping technology and computerized collision diagramming software for collision scene analysis. Al is experienced in the data download and analysis of airbag black boxes (Crash Data Retrieval Units) in automobiles, pickup trucks and SUVs. He holds a Bachelor of Science Degree in Political Science from Lander University in Greenwood, South Carolina and completed the Law Enforcement Basic Program at the South Carolina Criminal Justice Academy in Columbia, South Carolina


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