WHY Use Simulation to “Test and Refine” Traditional Crash Reconstruction Results

[MSI]

WHY Use Simulation to “Test and Refine” Traditional Crash Reconstruction Results

This topic is a follow-up on some other related topics:

• Should an Expert Rely Solely on PC-Crash for an Opinion?
• Simulation Analysis: Effects of Input Variations
• Classic Example of Sensitivity of Momentum Solutions

The main point of WHY we say TEST and REFINE traditional methods is to REMIND crash reconstructionists that:

• It is EXTREMELY important that you carefully examine ALL the evidence, all the details, damage, scene evidence, police reports, etc etc

BEFORE turning on the ANY simulation program.

Simplified Momentum Solution Procedures like Programs like PC-CRASH, Virtual Crash, Planar Impact Models and other Momentum based solution procedures mainly update their software to have more fancy graphics so you can make ANYTHING look real!

• Is that scientific crash reconstruction?
  • For additional info and our suggestions for refinements see:
    • 3D or NOT 3D? THAT is the Question!
    • Recommendations for Refinement of Simplified Momentum Solution Procedures

In a recent case in past year or so a rebuttal report by an expert, who exclusively and ONLY used the program PC-CRASH, to reconstruct the crash in his main crash reconstruction report. He later wrote a rebuttal report to our crash reconstruction report critical of his 'simulation only' analysis.
In our report we pointed out the many missing elements of the expert's crash analysis and reconstruction, and their arbitrary conclusions about accelerations and speeds, which they based ONLY on the results of a flawed PC-CRASH analysis.
Here is a sampling of the expert's rebuttal statements:

• "The true test of whether simplifying the collision model as an instantaneous time rather than a time of 0.1 to 0.2 seconds comes in the validation of the software. PC-Crash has been tested 2 and validated in a number of publications. No shortcoming has been identified associated with the simplifying assumption."
  
  • See our topic Validation Notes on PC-CRASH Simplified Momentum
    • NOTE: Calculate the individual case errors on their validations and you will find that they are >20% in several cases
      How do you know if the case you are investigation falls within that problem area?
      And this is with them KNOWING the correct answer while doing their validations
  • In our report in that recent case we also stated:
    • PC-CRASH, Virtual Crash and other Momentum based solution procedures related to accident reconstruction should be used to test and refine the results of a conventional accident reconstruction analysis, not create the sole basis for conclusions..
      
      • The experts response was:
        
        • "I disagree with this statement. Students in school are taught long division, but few use long division these days. Fewer still will do long division before then testing and refining the results with a calculator. If a person new to this field has never performed momentum analysis of a collision manually then it is certainly important to learn how to solve this both numerically and also graphically with a vector diagram. However, to perform hand calculations on every case is a waste of time, and an unnecessary inaccurate first step. Simulations allow one to explore a wide range of inputs in a much more complex case far more quickly and accurately".
        and also
        • "One should note that Mr. McHenry states that PC-Crash can be used to refine. the results of a conventional analysis. This implies that the results of a conventional analysis can be improved upon through the use of a simulation. This in turn suggests to me that he is stating that higher accuracy can be achieved with a simulation. I would agree with that."
          
    OUR RESPONE:
    • NOTE: The expert left out the "TEST" of my statement "TEST and REFINE"!
      • Simplified momentum solutions CAN BE VERY SENSITIVE
        • Testing is to emphasize that It is EXTREMELY important that you carefully examine ALL the evidence, all the details, damage, scene evidence, police reports, etc etc BEFORE turning on the computer.
      • TESTING and REFINING means checking if changing any of the sometimes arbitrary and subjective inputs can significantly change the results of the simulation program.
        • This expert did not do that in this particular case.
      • As we have also mentioned in several posts a LOT of our consulting business includes simply getting the ‘FINAL’ crash simulation results of Pc-Crash, Virtual Crash and other Momentum based solution procedures and making small simple changes to the inputs
        • Like minor changes to the arbitrary and subjective "point and angle of momentum exchange" and we can "prove" with their simulation tool a contrary conclusions.
        • Recall also a judge in a Canadian ruling in a case where both opposing experts used pc-crash and came to opposite conclusions, the judge stated the following:
          • "For the reasons that follow, EXPERT NAME REDACTED evidence, in light of the above, is of no assistance to me."
    • And in the rebuttal report, the expert concluded:
      
      • “I am confident that PC-Crash is a reliable and accurate tool when used by a skilled and knowledgeable user"
      
      One must wonder does ‘skilled and knowledgeable user’ simply mean someone who can manipulate the program to give them the results that they want and need?
    We will update this topic with more detailed information in the near future, perhaps a redacted version of the reports, so stay tuned!