Introduction: Accident Reconstruction Techniques and Accuracy

by Brian McHenry

(Originally presented at the NHMRC Road Accident Research Unit (RARU), Adelaide, Australia in May 1995)

This is also found in our Forum


The accuracy of any reconstruction technique, including computerizedtechniques, is highly dependent on a number of variables:

  • Some of the variables are related to the preservationof the Collision Evidence,
  • Some are related to limitations in Available Specifications,
  • Some are related to the choice of Collision Reconstruction Techniques which fall into two categories:

Accident Evidence

The first group of variables related to a collision reconstructionare classified as collision evidence which are the facts of theparticular case; they are unique for a particularcase and should only vary between cases. These items arenot under the direct control of a collision reconstructionist except withrespect to the reconstructionist’s request for their documentation and/ormemorialization. These form the basis for the reconstruction.

The collision evidence, once preserved, should not vary for a particularcase. If not preserved or memorialized in photos ormeasurements by police or other investigators at the time of the collision,some of these facts of the case may be forever lost and unavailable to the collision reconstructionist.Their absence detracts from the attainable accuracy of any reconstructiontechnique.

The items which fall under the category of collisionevidence include positions of rest, tire marks, roadway markings, damageto vehicles, and damage to property. The memorialization of these itemsmay vary widely between cases. First the collision investigator (normallythe police) must identify the important aspects required to permit a detailedreconstruction. The determination of the requirements of a reconstructionmay occur while many more important things are being handled, like life-savingand the restoration of a safe environment to the collision site. The investigatormust try to preserve as much of the evidence as possible. The use of photographyin conjunction with paint markings to preserve the vehicles positions ofrest, impact marking and debris might be the best technique until a senseof order is restored and a more thorough technique can begin. Of course,in many instances photographing a specific item of evidence before puttingpaint marks on it should be the preferable choice.

There is also a wide variety in techniques used to measurevarious items at the scene, varying from simple use of sight estimatesand pacing of the approximate distances to the more accurate use of tapemeasures and even possible use of surveying type equipment. The variationin the accuracy of these techniques may detract from the ultimate accuracyof the speed estimates.

The vehicle damage data does not necessarily need to bepreserved at the scene. Except for the possible use of extraction devicesat the scene to remove the vehicle occupants, and/or additional damagewhich may occur during transit and/or at the storage facility, normallythe vehicle damage will remain unchanged for weeks and/or years at a separatelocation while either waiting for repair or disposal. This may accountfor the wide popularity of the use of damage only collision reconstructiontechniques by many collision investigators. The use of a damage-onlybased collision reconstruction should be considered a last resort techniqueto be used only in the absence of site data. Normal reconstruction techniquesshould include the consideration of both damage and scenedata for a comprehensive reconstruction.

The measurement of the extent of vehicle damage is alsosubject to some variation. The variation of results of a damage-data basedreconstruction is mainly due to differences in the reconstruction and interpretationtechniques utilized than to the measurement devices used. The results fora generally competent measurement technique with string, level and measuringtape, compares favorably with those obtained by the use of survey-typemeasurement equipment applied with similar damage-based reconstructionequations.

It should be noted that many papers have been dedicatedto the detailed measurement techniques and guidelines to properly documentthe damage produced to a vehicle as a result of a collision (e.g., Ref33). Some of these guidelines are to account for recognized shortcomingsof damage-only reconstruction techniques. For example, many reconstructionistsinclude induced damage in their crush measurements in general and/or fornarrow object contacts, particularly pole impacts, to compensate for shortcomingsin the damage-based techniques. Consistency of measurement techniques isvery important, particularly for case studies involving many collisions.Although the results may ultimately fall short of the desired accuracy,use of a consistent measurement protocol will lend itself to later correctiveadjustments as more advanced techniques evolve.|


Available Specifications

The next item to consider in the preparation of a reconstructionis the vehicle and other specifications. These form the numberswhich go into the equations to permit applications of Newton’s Laws forthe reconstruction. As a brief review, Newton’s Laws of motion are:

Newton’s Laws of Motion

      1. Every body continues in its state of rest or of uniformmotion in a straight line, except in so far as it is compelled by someexternal force to change that state.
      2. The acceleration of a body is in the direction of, andproportional to, the force that produces it, and is inversely proportionalto the mass of the body.
      3. To every force there is an equal and opposite reaction,or the mutual actions of two bodies are equal and opposite.

As you can deduce, the determination of the mass of thebodies impacting and the geometry of the collisions ( which requires thedimensions of the vehicles ) may also add a variable to the final resultingaccuracy of the reconstruction technique. Various sources exist to approximatethe vehicle properties, however there always will be some amount of variabilityin the results due to minor differences in the sources of vehicle propertiesand calculation techniques for those inputs which must be approximated.Again, particularly for case studies, consistency of technique must bestressed to insure that the results will be consistent and lend themselvesto possible future adjustments and refinements.

Other Specifications required for a reconstruction includeapproximation for roadway friction coefficients, wheel drag and wheel steer.These are used primarily for trajectory-based analysis where thefriction coefficient, drag and steer on the vehicle as it travels fromimpact to rest are required to permit approximation of the kinetic energydissipated.|


Collision reconstruction techniques

Now that the investigator has all the items required theinvestigator must chose which collision reconstruction techniqueto use. The techniques for collision reconstruction vary widelyin levels of sophistication and underlying simplifying assumptions. Thechoice of any technique will have related strengths and weaknesses. Forthe collision reconstructionist to obtain the most accurate reconstructionrequires a careful evaluation and choice from the techniques available.The two general techniques for collision reconstruction are either damage-basedor trajectory-based: |


Damage-based reconstruction techniques

During the late ’60’s and early ’70’s, a number of technicalpapers in the field of highway safety indicated that reasonably accurateestimates of the speed-changes that occur in wide-contact collisions canbe obtained through the use of simple linear relationships between theimpact speed-change and the extent of residual crush (e.g., References 50-53, 3). While such relationships obviouslyconstitute a gross simplification of complex automobile structures, theywere found to be capable of yielding impact speed-change results with approximatelya plus or minus 10 percent accuracy when applied to a limited number ofstaged collisions (e.g., Reference 46). In a morecomprehensive evaluation (Reference 54), it wasfound that the 95 percent confidence limits on individual calculationsof delta-V ranged from 9 to 25 percent.

The current state-of-the-art of damage-based onlyreconstruction techniques are inadequate for individual case reconstructions.The basic premise of damage­based only reconstruction techniquesis an assumption of a linear delta-V vs. residual crush “virtual“relationship (e.g., Ref 34, 17,etc.) (The term “virtual” is used to emphasize the factthat the crush energy is dissipated during the dynamic crushing of thevehicles and that equating the residual (restituted or static) crush tothe energy dissipated is a “virtual” relationship, i.e.,they do not occur simultaneously). Many collision reconstructionists usea single full-scale crash test data point for a given vehicle, combinedwith an assumption regarding a “no­damage” intercept, tocalculate custom-fitted coefficients for use in individual case reconstructions.This must be recognized as a crude “first-approximation” procedure.Any suggestion that the resulting coefficients constitute a reliable definitionof the structural crush responses of a given vehicle is misleading andnot in keeping with sound engineering practices and principles. Problemsassociated with the repeatability of full-scale crash test results whichproduce scatter, limit the reliability and accuracy of a single data point.Minimum scientific principles would require an absolute minimum of at leastthree data points. In response, some researchers are using techniques involvingmultiple crash tests on an individual vehicle to create multiple data pointsfor a given vehicle (e.g., Ref 35). The problemsassociated with the reliability of the crash test procedure, the differencesin the crush response characteristics of a pre-damaged vehicle vs. a “fresh”undamaged vehicle and the underlying assumption of the linearity of the”virtual” relationship of delta-V vs. residual crush stillclassify the damage-based only reconstruction as a crude “first-approximation”technique

For additional information please see our Reviewof CRASH damage analysis including a discussion of the NHTSA”reformulation” by Prasad and our recent SAE paper 97-0960- Effects of Restitution in the Application of Crush Coefficients.|


Trajectory-based reconstruction techniques

The generalized concept behind trajectory-basedreconstruction is as follows: On the basis of Newton’s 2nd and 3rd laws,the total momentum of an isolated system of masses remain constant. Thisprincipal, which is referred to as the Conservation of Momentum, servesas the theoretical basis for reconstruction of impact speeds in vehicle-to-vehiclecollisions. The principal stipulates that the system momentum precedinga collision and the system momentum after a collision, e.g. at separation,are conserved in the absence of external forces. Therefore, if we can determinethe individual speeds and directions of motion that are required for eachof the two partners in a collision to travel from separation to rest, thenthe direction and magnitude of this system momentum can be used to determinethe magnitudes and directions of the velocities which must have existedprior to the collision, the impact velocities. (The magnitude of externalforces produced by the tires and other possible sources (such as gougingand scraping of vehicle components on the ground) during the collisionare normally small when compared to the magnitude of the forces of thecollision, however they should not be totally ignored for a comprehensivereconstruction).

When vehicles separate after a collision, they move torest positions against resistance forces produced primarily by tire-groundfriction. Secondary contacts may also occur with roadside obstacles and/orterrain features which sometimes can play significant roles in the dissipationof kinetic energy and can also produce redirection of the spinout trajectories.The task of analyzing the total energy dissipated as the vehicles travelfrom separation to their positions of rest is essential in preparing acomprehensive trajectory-based reconstruction of the collision.
A trajectory-based analysis directly provides estimates of the impactspeed-changes (delta-V) in the form of the differences between impact andseparation velocities for each vehicle.

For additional information on trajectory analysis proceduresplease see our recent SAE paper 97-0949 – CRASH-97- Refinement of the Trajectory Solution Procedure |


Summary

In summary, the choice of an collision reconstruction techniqueshould always use all available information, both vehicledamage and scene information. The utilization of a particular techniquefor convenience and/or due to assumptions of consistently small averageerrors should be carefully scrutinized. Wherever possible, a techniquewhich includes both scene and vehicle information should be usedto perform an collision reconstruction. The process of reconstructing acollision must be seen as first collecting from the scene, police reportand photos collision evidence, next obtaining from data sources availablespecifications, and then choosing an collision reconstruction techniqueto perform the best possible reconstruction for the data collected andtime available.

For additional information on accident reconstructiontechniques, please see our recent SAE papers on trajectory and damage analysisprocedures:

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