PC-CRASH: How Many % May An Investigator Be Wrong with pc-crash?

General Questions related to the Momentum Based Analysis programs
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MSI
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PC-CRASH: How Many % May An Investigator Be Wrong with pc-crash?

Post by MSI »

In a Crash Forum there was a posting desperate for answers which included a description of a crash in Europe or other country and included the question:
  • "how many % may an investigator be wrong with pc-crash?"
    • The desperate for answers author also relayed that:
      • The police used pc crash which they said was PROOF that their daughter was coming from the right, from her lane...very slowly...
      • A second expert was hired who made a little video [impact to rest only) from pc-crash which was the same scenario as police
      • The 2nd expert never tested their daughter's version of the crash with pc-crash or other means...
      • They later found out that the 2nd expert was the regular expert from the assurance company
        In summary neither the police or the 2 experts ever tried any alternate theories of how/why the crash happened
We initially posted this to our post HELP! If YOU or someone you know has been in a crash! However a few other related questions came in so we made this also a separate thread and added additional detail:
  • Note that the crash outlined by desperate for answers was a 'who's on the wrong side of the road' question and so we included an example where pc-crash was used improperly as a basis for an incorrect opinion in a case.
  • This topic/response was prepared to assist anyone in event a police department or expert has used and presented PC-CRASH (or other simulation program) as PROOF of a theory...PC-CRASH is a very widely used software product however we have found that it can be easy to manipulate the results sometimes to support a preconceived opinion. Any PC-CRASH (or other simulation program) PROOF (and videos created as a result) need to be carefully examined as they might be a smokescreen.
  • All crash reconstruction and simulation software products are interesting and useful products however as with ANY product users must exercise caution when using and be sure to check the veracity of any results.
  • We always recommend that crash reconstruction simulation software be used to test and refine opinions which also sometimes means testing alternate theories.
The following is our response (with some additional information added to this thread) and anyone seeking an experts help in a collision may benefit from this list:
  • Where are you located? Are there not any local experts who can help?
    • If you provide where you are located perhaps someone on this forum knows someone in that area.
You provided a narrative however you did not provide any evidence (pictures, diagrams, reports) on which to offer you any assistance. The following is a list of items which you should provide to allow for a proper evaluation of the crash.
  • Pictures of exterior damage to the vehicles.
  • Pictures of the vehicles at the scene.
  • You mentioned drone pics? post those
  • A police report? Any crash scene diagrams? post those
  • The location of the crash? Google maps or Google Earth can be helpful in determining distances, etc.
You mentioned:
  • "how many % may an investigator be wrong with pc-crash?"
Did the police or some expert in your case use pc-crash?
  • if so, first the answer to that question is 'it depends'.
    • pc-crash is a momentum based simulation solution and so subject to momentum limitations and simplifying assumptions
      In several cases we have been able to make a singular change in inputs (the point and angle of momentum exchange') and change the results of a pc-crash simulation used as the basis for an opinion by an expert in a case.
      One example was a 'who's on the wrong side of the road' crash (see below)
You should obtain the inputs and the diagrams from any pc-crash simulation (or ANY computer program/simulation) used in your case.
You should also demand you get the input file in electronic form for ANY simulation/animation so you can have an expert review all inputs and rerun the simulation to verify that the inputs produce the results and then also to allow a test for sensitivities of inputs.
  • A demonstration of sensitivities of momentum based solutions comes from How is Monte-Carlo method used in Accident reconstruction?: What that ‘classic’ example is meant to illustrate is that when applying a Linear Momentum solution procedure to ANY accident you need to test sensitivities of inputs (angles at impact and angles at separation). If a small change in an angle makes a dramatic change in the results then obviously you need to focus on defining and refining the inputs as well as consider using a more sophisticated solution procedure (like a SMAC simulation).

    All simulation inputs for pc-crash may not be included in a printed run summary.
    • In most courts in the USA the inputs for any simulation or animation must be provided in electronic form.
      This allows the other side to rerun the simulation to be sure all inputs are produced so they can be checked
      • weights? specifications? tire properties? values for friction? friction zones? terrain slopes? source of terrain information?
        • Today with cloud scanning of scenes a reconstruction (by Leica, Trimble and/or Faro) can include gigabytes of information about the crash scene and vehicles which are NOT included in any printout (or is they are it will be thousands of pages of unintelligible numbers)...not quite full disclosure of inputs. again WHY full input file in electronic form MUST be required.
      • Proper dimensions for evidence location and ability to compare that to the simulation/animation?
      • if animation, is the animation physically possible and does it obey Newton's Laws?
        • the electronic inputs provide a list of the positions and orientations for each frame so the speeds, speed changes, and approx accelerations can be calculated
      • And do the graphics/video created match the inputs provided?
        • we have cases where inputs (in paper form) and graphics/video provided looked OK however once we obtained in electronic form we found that the printed paper inputs were different than the electronic inputs and did NOT produce the same results/video/graphics!
    Here are some links with additional information: EXAMPLE: Here is a comparison of two pc-crash runs in a case.
    • An expert used pc-crash as the basis for his expert opinion of who was on the wrong side of the road.
      Once we obtained the electronic pc-crash inputs, we moved the impact point of the vehicles and changed the arbitrary and subjective 'point of momentum exchange' and also 'matched the evidence' (see below)
      We demonstrated that either scenario could be proven with pc-crash.
      So what to do?
      We then tested the experts 'proof' by setting up and running the crash with msmac which produced very different results for the two scenarios. Since with SMAC the solution includes the trajectory and damage produced by a given impact configuration and speeds.
      • msmac is like running a mathematical full scale test since it models the collision forces for each millisecond the vehicles structures interact (100-150 milliseconds) instead of trying to approximate the results of a collision interaction with an "instantaneous exchange of momentum" at an "arbitrary point of momentum exchange"
        • With msmac you set up the mathematical full scale test, set the speeds and then hands off run (simulate) the vehicles into each other. It calculates the movements of the vehicles and displays the structural damage that occurs from the impact speed, relative locations, movements of the vehicles and relative crush stiffnesses (it calculates the vehicle trajectories and collision forces for each and every millisecond!).
        • With pc-crash you must pick a 'point of maximum engagement'' and move the vehicles to that position with no feedback or verification as to whether the speeds, relative locations and collision interaction of the vehicles will produce damage which in any way matches the amount and location of the actual damage. The collision forces are not calculated.
      • We say pc-crash as an arbitrary and subjective 'point of maximum engagement and angle of contact plane' because the pc-crash input guidelines and training videos require the user to subjectively position the 'point of maximum engagement"
        As the pc-crash user moves the point and angle around the user can watch the speed results change until an arbitrary and subjective position and angle is found which produces the desired or known results. Note: there is an 'auto calc' option on their crash simulation dialog however there is no indication if it has been used. In every case we have been asked to evaluate a pc-crash reconstruction it has NOT been selected and selecting it and rerunning the collision dramatically changed the results.
    Pc-crash, and other momentum or planar impact based collision programs, can be easily manipulated. So be sure to test and check any simulations which form the basis for conclusions.
    And of course test and check ANY reconstruction, simulation or animation program results.


    The experts pc-crash reconstruction which "proved" a vehicle on the wrong side of the road was as follows:
    wrong side 2.png
    wrong side 2.png (464.91 KiB) Viewed 8970 times
    However we then with pc-crash put the vehicle in its proper lane and made a minor change to the arbitrary and subjective pc-crash 'point of momentum exchange which then also "proved" that the vehicle was in its proper lane!!
    wrong side 1.png
    wrong side 1.png (530.54 KiB) Viewed 8970 times
    MAIN POINT: Pc-crash, and other simplified momentum based collision programs, can be easily manipulated. Be sure to test and check any conclusions based on momentum based programs or equations. Test and check ANY and ALL reconstruction results.

This topic has 6 more posts with additional information

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