An Accident Reconstruction Primer

Traffic accidents are the elephant in the room				police investigation is to our work, nor how often
nobody wants to talk about. We go about our				it is not taken seriously by the investigating
daily lives without thinking for one moment about				agency. It is hard to believe by the uninitiated, but
what might happen if we are involved in a serious				an untrained or unmotivated officer is not held
crash, because we always believe that our vehicle				responsible for any errors, mistakes, or flat out
will not be the one that is being craned out of a				wrong information that may be in a TCR unless it
deep ravine, or loaded onto a flatbed truck in a				can be proven that it was intentional. Sadly,
heap of twisted metal. Unfortunately, statistics				careless or slanted accident reporting is simply
suggest it is more likely than not that some who				accepted as the status-quo by most police
read this will be involved in a serious accident at				agencies. This is not to say that I do not see
some time in their life. Further, in our increasingly				very good, well documented, well researched, and
litigious culture, there is exists a very real potential				excellent police investigations; but it is unfortunate
that we will become involved as plaintiffs or				to accident victims that conscientious police
defendants in a civil or criminal action resulting				reporting is by far the exception rather than the
from a car accident. When this happens, you				rule.
should hire the best attorney you can afford, and				Our accident plot usually includes scale
preserve as much evidence as you can. Every				representations of the vehicles in their positions at
driver should understand a little about what makes				the point(s) of impact, points of rest, and any
up a typical accident study, if only to protect his				physical evidence associated with the accident
or her own interests in the aftermath of a terrible				superimposed on the scale diagram. This allows us
collision.				to measure specific items like the distances
Generally speaking, accident reconstructionists are				traveled before and after impact, the approach
professional experts and consultants who are				and departure angles of the vehicles, the
usually retained by legal advocates. We are				deceleration forces that influence a vehicle (skids,
oftentimes engineers, physicists, ex-police officers,				gouges, etc.), and any other relevant facts with
or highway safety professionals, but we are				respect to the physics of the crash. This step is
specialized by experience and training in the				of critical importance because it provides the basis
analysis of traffic collisions. Though we usually				for everything that follows. To wit, it is very
work for the legal advocates defending or				difficult to evaluate the deceleration of a spinning
prosecuting a particular case, we are independent				vehicle without a good accident plot because the
of any particular "side" in a legal action. The story				forces compelling that vehicle to slow are
of each accident study is different, but the				transient and are a function of what is known as
efforts accident reconstructionists travel along				the slip angle. The only way to know the slip angle
well-worn paths to eventually arrive at our expert				at a given point in time or space is to know
opinions.				where the vehicle came from, where it is going,
At the first consultation with a potential client we				and where it is now, based upon the physical
may reject a case because it does not appear				evidence. All of the information obtained during
that we can offer any help (e.g. their client is				discovery has its appropriate value as it is
clearly at fault, there is no legitimate accident to				examined within the context of a traffic accident,
reconstruct, the deadline is too soon, etc.).				but how much value it has requires some
Sometimes initial contact is made at the accident				subjective analysis on the part of the
location so that the issues and circumstances can				reconstructionist.
be better understood and an informed decision				Calculations. Usually we will be asked to determine
can be made as to whether or not we should				vehicle speeds in a given accident, but this is not
become involved. Accident reconstructionists try				always possible from the data provided. There
to avoid being compelled to render opinions				are various methods we employ to make this
contrary to our client's case.				determination. Some methods are empirical,
Determine the facts. Usually the first evidentiary				meaning they are based upon comparing the
item found in a typical file is the traffic collision				subject accident to physical models that have
report (TCR), as they are called in California;				been previously developed by controlled testing.
elsewhere they have other names. This				This is how we usually evaluate damage energy,
documents the police investigation into the				vehicle deceleration when we cannot test the
accident and usually provides most of the factual				actual surface friction, and other points that we
information necessary to conduct an analysis.				have been asked to address. Other methods are
Although some accidents are more thoroughly				theoretical, which is to say they are based upon
investigated by law enforcement than others,				the theories and principles of physics and
most reports identify the parties and their				mathematics, rather than documented
vehicles, provide witness statements and contact				observations from testing. That is not to say that
information, offer some description of the				theoretical methods have not been tested, but
accident scene as it was found by the				that they were not developed from testing. In
investigating officer, and may contain some				accident reconstruction, non-empirical methods are
measurements and/or diagrams that describe the				derived from Newton's Laws of Physics and from
locations of various items of physical evidence.				his formulas of uniform motion. Higher order math
This evidence might include any tire marks,				is employed to be sure; however, nearly all but
gouges, dirt tracks, debris, and the vehicles' points				the most esoteric theoretical analyses can be
of rest. Hopefully the investigation includes				boiled down to Sir Isaac Newton's observations of
photographs of the scene, which are critical to				our physical world. To simplify the concept,
any traffic collision report. It seems that police				empirical methods employ inductive reasoning and
officers do not always think so because only				theoretical methods employ deductive reasoning,
about 50% of the crashes I investigate include				but they both can help us understand the physics
scene photographs, and less than half of those				of accident reconstruction.
cases include useful photographs because of the				Some of the methods that are typically used to
low priority most agencies put on accident				estimate speed include the theory of the
investigation training. This is a terrible shame				conservation of momentum, conservation of
because peoples' memories fade with time, but				energy, damage energy estimates using crash
photographs can preserve evidence that was not				test data from the National Highway
collected at the scene of the crash or memorialize				Transportation Safety Administration (NHTSA),
temporary conditions that no longer exist.				and the myriad forms and permutations of
If you are ever involved in a traffic accident, the				Newton's particle kinematic equations (I have
best advice I can offer is that you do not provide				even derived a few new theoretical formulas
a statement to the police at the scene. This may				using these equations that are useful in certain
seem a little strange coming from a person who				situations). Sometimes, due to a lack of the
relies heavily on the information presented in the				appropriate data, there is no other way to
police report, but it is in your own best interest to				estimate a vehicle's speed in a crash than to
provide a written statement at a later time.				consider the witness statements in the police
Immediately following an accident, emotions are				report, or review the available deposition
high and a person might say inaccurate things				transcripts of percipient witnesses, and arrive at
because they feel guilty or feel bad for another				an estimate based upon the entire pool of
party who was hurt. I have never read a				knowledge one develops only through experience
statement that did anything to help the person				in crash investigation.
making it, but I have read hundreds that led to				Sometimes it is not important to calculate the
their arrest. I suggest a written statement				speed of a vehicle at all because it has been
because police officers are humans who cannot				stipulated to, or because it is irrelevant to the
be expected to paraphrase your statement				immediate question at hand. It might be that what
perfectly. Any differences between your				we really want to know is the time elapsed
statement and what eventually is recorded in the				between events, or the distance a vehicle was
report can completely change the meaning of				from impact at the point in time that another
what you said. Once the officer has filed his				party looked to see if it was safe to proceed. On
report, the statement cannot be taken back so				other occasions, usually for highway design
choose your words very carefully.				evaluations, we are more concerned with the
Site Investigation. After we have the location and				operational speed or design speed of a roadway,
some details regarding the issues we have been				so that we can evaluate the roadway using more
asked to evaluate, we visit the accident site to				objective criteria before comparing it to the
conduct an independent topographic survey. We				subject accident. For example, if a roadway is
conduct a survey as it would be conducted by a				designed for 65 mph traffic and we calculate that
traditional surveying firm with the obvious				at 65 mph a car will loose traction around a curve
exception that we pay particularly close attention				on that roadway, then the curve should be
to any remaining physical evidence. The site is				engineered to reduce the high potential for
photographed comprehensively and the survey				run-off-road and head-on accidents at the
might also include surface friction tests,				deficient curve (e.g. curve specific signage,
sight-distance measurements, video, radar speed				reduced speed limit + enforcement, increased
surveys, illumination level readings, ball-bank				illumination, advanced warning, etc.). In this
measurements, and any other tests requested by				example we do not need to know what the
the client or that we feel will help us in our				speed of a subject vehicle was to evaluate the
investigation. Sometimes when the accident site is				potential for safety problems at such a location.
in another state or if we are severely limited by				Whether or not an analysis like the example is
time, other surveying firms are contracted to				relevant to a particular accident situation is
provide us with topographic measurements.				another matter altogether and that question is
However, most accident reconstructionists prefer				sometimes answered only through a synthesis of
to conduct their own investigation at the accident				concepts from the fields of accident
site.				reconstruction, law, psychology, kinesiology,
Vehicle Inspection. If the subject vehicles have				automotive engineering, highway engineering, and
not been destroyed or repaired, a				logic.
reconstructionist will inspect them to collect				Conclusions. Now that everything has been
evidence that is helpful in most studies and critical				brought together and thoroughly examined, we
in product liability lawsuits. Inspections can occur at				present our findings and recapitulate the facts
any time as the vehicles become available. My				that are most important to our analysis. This is
personal preference is to see the cars before the				probably where most reconstructionists will have
field review because sometimes questions arise				disagreements between them. Depending on the
during the vehicle inspections that can be				hierarchy of facts, two analyses of the same
answered by collecting evidence at the site.				accident may have different conclusions. If more
Without the clues sometimes discovered while				weight is given to witness testimony and less to
examining the vehicles, I might miss the				the police measurements, one conclusion may be
importance of something unusual or unique at the				presented. If the opposite is considered, another
accident location.				conclusion can be opined. It is not uncommon for
Inspecting the vehicles can provide clues to the				the ultimate hierarchy of facts to be determined
severity of the impact, the occupant motion, the				by a judge, legal referee, or the people asked to
crashworthiness of the vehicle, seatbelt usage,				sit on a jury in a court of law. Few of these
airbag performance, and may implicate the vehicle				individuals are scientists, and even fewer are
itself in the cause of the accident (e.g. tire				accident reconstructionists or highway engineers.
puncture/blowout, electrical fire, brake failure).				They rely on the opinions and conclusions
Most importantly, this information can sometimes				presented by experts on both sides of a case
be preserved by the owner of the vehicle				and must weigh which represents a more
through photographs. Any time you are involved				reasonable reconstruction of the events in a
in a traffic accident, you should take as many				traffic accident. The presentation of opinions is
photographs of your vehicle as you possibly can				therefore very important to an accident study,
before it is destroyed or sold for parts. The value				and the best scientific analysis by the research
of these photographs cannot be overstated, and				teams at JPL is worthless if it is not understood
one should not rely on 6, 8, or even 12				or believed by the people who must ultimately
photographs to tell the story. Film is cheap and				decide on its validity.
digital prints are even cheaper, so if you are				Sometimes the best analysis is not always the
physically able to do so, do not hesitate to take				most accurate because automobile accidents
one-hundred or more photographs of your				rarely require extreme precision. Conclusions are
crashed vehicle.				often presented that consider ranges of speeds
A majority of vehicle inspections concern the				or distances that make very precise calculations
recording of certain measurements that will later				impractical. For example, it makes little difference
allow us to estimate the energy that was				to most people if a vehicle is traveling at 54.2
dissipated by the crushing of metal and				mph or 56.1 mph, because they would say that
deformation of structure that occurred at the				car is going 55 miles per hour. Furthermore, I
time of an accident. Most people intuitively				cannot think of a case where such a fine
understand that the greater the level of				distinction could be discerned by the person in the
permanent displacement to a vehicle's form, the				car (or anyone else) at the time of the accident
greater the effort must have been to displace it.				so what relevance could it possibly have on its
In strictly Newtonian terms, if we consider				cause? The obvious conclusion is that precision is
crash-related damage to be the result of work				important, but clarity and simplicity have their
that is done at the time of the accident, and that				place in determining the hierarchy of facts that
the energy to perform the work is provided by				lead to a valid reconstruction.
the mass and speeds of the vehicles in the				Sadly, it should be mentioned that there are
collision, we can develop formulas to estimate the				reconstructionists and engineers, driven by
amount of energy that was required to do the				notoriety or money, that sacrifice their scientific
work. If we can determine a reliable estimate of				integrity to achieve selfish ends. Sometimes
this work energy, we can draw conclusions about				reconstructionists are victim to the otherwise
the speed change that a particular vehicle may				altruistic desire to do the best job possible for
have experienced as a result of the damaging				their clients, and can become blinded to the true
event. This might ultimately lead to some				hierarchy of facts. Other times, inaccurate or
conclusions regarding the approach speeds of the				disingenuous conclusions are presented by experts
vehicles.				who would rather not be bothered by the truth
Discovery. For a period of time, which varies by				when it conflicts with the claims made by their
statute and the jurisdiction of the case, the legal				"side" in a legal action. At one point or another
minds conduct discovery and provide us with				every expert is presented with a difficult situation,
other forms of investigative information. The				and it is the character of the individual
discovery material often includes depositions of				reconstructionist or engineer that determines how
percipient witnesses to supplement statements				they will respond to it. As an individual accident
from the TCR, depositions of the appropriate				reconstructionist, and as part of Krueper
highway officials that have knowledge about the				Engineering and Associates, Inc., I try to always
history and design of the roadway, depositions of				be conscious of the greater responsibility
other experts retained by various plaintiffs and				consultants have as human beings and as trusted
defendants, photographs of anything related to				experts to present opinions and conclusions that
the crash, and basically any information not part				we believe to be grounded in a pursuit of the
of the initial police investigation. Discovery can				truth, and by extension, the pursuit of safer
sometimes drag on for years during which				ground transportation. Sometimes the truth is that
information trickles into our office in bits and				a person has, whether through the actions or
pieces. We continue working during discovery and				inactions of people or governments, been
revise our opinions and conclusions, if necessary,				damaged in a crash; and they should be
as new information is received. Our investigatory				appropriately compensated. Other times we may
services are considered part of the discovery				conclude that a party was driving without
process because the reports and opinions we				appropriate care and is responsible for their
produce for our clients, with certain exceptions,				injuries (and perhaps others' injuries as well).
become part of the public record and must be				Rarely is it so black and white because frequently
provided to any party who asks for them.				the truth lies somewhere in between the claims
Accident Reconstruction. Once we have gathered				made by both sides in a legal conflict. Most
sufficient discovery, identified what we are to				accidents are the result of many disperse and
evaluate, and have been given authorization, we				seemingly unrelated circumstances that converge
can finally proceed with the accident				at a particular time, in a particular place, and
reconstruction. Usually this begins with developing				ultimately result in a serious traffic collision.
a diagram of the accident site based upon our				This is what accident reconstructionists and
survey, the TCR, and any photographs of the				highway safety engineers do. We pull all of the
scene taken at or near the time of the accident.				information and data from every source available
When I first started in the field, before				to determine what the circumstances were, why
micro-computers were commonplace, I used to				they came together as they did, and how they
draft everything by hand with technical ink pens.				are relevant to the cause of a crash. Most
That was quite time consuming and with				importantly, an opinion is almost always offered
advances in technology I am happy to say most				that describes ways that the accident might have
no longer draft using technical pens and lettering				been avoided, or made less severe for the
guides. I do all of my diagram development using				injured parties. As professionals, most
AutoCAD and various third-party add-ons to the				reconstructionsists feel that it is a matter of no
program that have greatly reduced the time				small importance that we make positive efforts
required to put a diagram of the roadway				to improve our collective driving experience.
environment together.				Whether through the advocacy of improved
Once there is a diagram, the accident can be				methods of roadway design and construction, or
plotted using the discovery information. This is				by holding a company, or government entity, or
where the police investigation becomes critical and				careless driver financially responsible for their level
its shortcomings and strengths will become				of causation in a traffic accident, we hope to
apparent. Since it is nearly unheard of for a				make vehicular travel safer with each accident
reconstructionist to visit the scene of an accident,				reconstruction and investigation. It is not likely, or
we must rely heavily on the police investigation				even possible, the investigation of traffic accidents
for accident scene data. If we visit the site within				might someday eliminate traffic accident injuries
a short time of the accident, we can record the				and fatalities. However, I believe that the majority
locations of evidence independently. When				of accident reconstruction professionals would
accidents occur on high speed expressways, it is				consider their lives a success, and gladly look for
oftentimes impossible to get this data without				new employment, if our services were no longer
shutting down traffic lanes, which almost never				necessary because traffic accidents had become
happens. I cannot express how important the				anachronisms of a bygone age.

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An Accident Reconstruction Primer