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

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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?"
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.
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?:
    Which Includes:
    • A classic example of problems with the sensitivity of a linear momentum solution is when it is applied to a t-bone type collision.
      • When a lighter car crosses the path of a heavier car/truck and the lighter vehicle is struck in the side by the heavier vehicle. If the heavier striking car/truck happens to swerve before the impact, either to the left or the right, the degree or two of change of impact angle can result in dramatic changes in the results of a linear momentum solution.
        Why?
        The swerve by the heavier vehicle will produce a change in the separation angles.
        The change in separation angles, if all attributed to the smaller vehicle speed (which it will be if the impact angle of the striking heavier vehicle is assumed to be 0 (zero) degrees) will dramatically change the linear momentum solution approximated speed of the smaller lighter vehicle.
        The result is that depending on the direction of the swerve, the small vehicle will be 'reconstructed' as either going very fast in the forward or reverse direction. Depending on the difference in the weights of the vehicles the assumption for impact angle of the heavier vehicle can result in very large errors in the analysis.
      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?
    • 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 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 209 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 209 times
Pc-crash, and other momentum based collision programs, can be easily manipulated. So be sure to test and check and conclusions based on a momentum based program.
And of course test and check ANY reconstruction or simulation program results.
Question? Comment? Please email forum@mchenrysoftware.com. Also see the McHenry Forum Index
Visit McHenrySoftware.com for technical information & software.
(c) McHenry Software, Inc ALL Rights Reserved.

MSI
Site Admin
Posts: 1386
Joined: Thu Jun 18, 2009 12:37 pm

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

Post by MSI »

In doing another reread of the narrative posted in the other forum and some pictures (some posted below) i notice mentioned:
  • "the other woman was driving on the wrong side and tried to correct to her side and the other driver on the right lane, tried to dodge, so the debris ended on the other lane and the driver who was riding on the wrong side and tried to correct was responsable..."
i am reminded of a driving story told to me by my late uncle Ken as a warning when driving...
  • One of his first driving experiences some 75+ years ago (he passed recently at 90+ years old) he was driving and saw a vehicle on his side of the road so he went to the other lane to try to pass the vehicle.
    The other driver at the last minute came back over to his proper lane and they collided.
    The police charged my uncle with being on the wrong side of the road and causing the crash.
    That is a problem since all the police can go by is 'where is the evidence' although the other driver was the 'town drunk' and drunk at the time (this was in rural Maine)
    Studies have shown that intoxicated people head 'towards the light' so that is why trying to swerve around someone traveling in your lane may make them come back 'to the light' and collide with you and make it appear you are to blame.
    it is also why at some crash sites all lit up that they 'attract' other intoxicated drivers who 'head towards the light' and crash into the rescue vehicles and cause further carnage.
The main point of this is that the police depend on the evidence and if all the evidence shows a collision in one lane, the person in the wrong lane is charged as your daughter was charged.

Why didn't the police download the EDRs in each vehicle?
  • you haven't indicated the make/model/year of each vehicle so they may not have information.
  • Then again some have 5 seconds prior to the crash.
  • As a minimum why didn't your expert read the EDR on your daughters vehicle since very easy process
You have not uploaded any more photos. In your narrative you mention pictures with police cars, etc
see link above for secure place to upload them.
And also if you have them, please send the pc-crash inputs and report and animation and any police and expert reports.
Unfortunately if the scenario you mention:
  • NOTE: This also still puts the white vehicle in the wrong lane but helps explain a possible reason why.
    • the other vehicle in your daughter's lane and so she swerved left and they crashed in the other vehicles lane the local law may make it your daughters fault even if you can prove the swerve left by your daughter was due to the other driver.
    As a quick demonstration, i did a msmac3D simulation of two somewhat similar vehicles on the scene w/pc-crash results you posted earlier in this thread
    In this quick preliminary demonstration i demonstrate if the other vehicle came back from the opposite lane you still can match the approximate rest positions of the vehicles
    • pc-crash includes a MAJOR simplifying assumption is an instantaneous exchange of momentum which relies on a user set arbitrary and subjective 'point of maximum engagement'
      • Actual crashes occur over 100-200 milliseconds with forces and moments acting during the entire crush interaction
    • If we were able to obtain the police/others pc-crash inputs (.PRO file and/or printed outputs) we probably could demonstrate that this scenario can also be "proven" by using a different arbitrary and subjective 'point of maximum engagement'
    • With msmac3D simulation you actually mathematically crash the vehicles into one another including calculating the forces and moments of the crush interface and interaction for every millisecond during the 100-200 millisecond crush interaction.
    The video below simply demonstrates the evidence might be matched in either scenario
    A thorough examination of the evidence might reveal which scenario is the best match of ALL the evidence best.
    • The weights, make/model/years, scene information,
      EVIDENCE/DATA REQUIRED IF AVAILABLE:
      • Damage photos/estimates/areas on each vehicle, extents
      • additional information is required to further refine a reconstruction
      • MAIN POINT one cannot simply state that pc-crash 'proved' a single scenario without examining another possible scenario and/or using a simulation program which Models the crushing of the vehicles and damage interaction to see if the two scenarios might allow determining which scenario best matches the evidence.
      • Adding the EDR data to the puzzle might also help clarify which vehicle did what pre-impact
        • if EDR data was available (make/model/year) from either/both vehicles it begs the question: WHY didn't the police read the EDR data?
    THE VIDEO BELOW IT IS A QUICK PRELIMINARY DEMONSTRATION!
    • NOTE: This also still puts the white vehicle in the wrong lane but helps explain a possible reason why.
      • the other vehicle in your daughter's lane and so she swerved left and they crashed in the other vehicles lane the local law may make it your daughters fault even if you can prove the swerve left by your daughter was due to the other driver.
      This is merely as a demonstration that the other vehicle might have been coming back into its proper lane.
      POI v POR msmac3D.jpg
      POI v POR msmac3D.jpg (238.16 KiB) Viewed 8 times
      See the Crash Forum and the 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?"

      here are a few of the pictures also posted:
      pdf photo received from the newspaper.jpg
      pdf photo received from the newspaper.jpg (21.6 KiB) Viewed 26 times
      photo expert defense.jpg
      photo expert defense.jpg (28.97 KiB) Viewed 26 times
      photo expert defense2.jpg
      photo expert defense2.jpg (28 KiB) Viewed 26 times
Question? Comment? Please email forum@mchenrysoftware.com. Also see the McHenry Forum Index
Visit McHenrySoftware.com for technical information & software.
(c) McHenry Software, Inc ALL Rights Reserved.

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