The "Big Eight": Take photos of all four sides
and all four corners of the vehicle. Even parts of the vehicle that appear
to be undamaged can be important.
Photograph the damaged area from at least three different
angles.
Take photos of the damaged regions with and without a
measuring stick in view. Make sure that your measuring stick has graduations
(of feet and inches) that can be clearly read in the photos.
Bring a step-ladder along. Get up on the ladder and take
some shots looking straight down on the damage (with your measuring stick in
view) so that the depth of the dent(s) may be measured.
Use the "landscape" view only (except in rare
cases where damage can only be viewed using the "portrait" view).
This makes it easier to view photos when they are placed in an album.
Be sure to take a photo of each of the following:
a. The license plate.
b. The VIN number (on the dashboard).
c. The vehicle identification tag (on the driver’s
side door).
d. The odometer reading.
Carry a disposable camera in every car you drive. Tell
your friends and colleagues to do this too. If you are involved in an
accident take as many photos as you can at the scene. Be sure to get photos
of all the vehicles involved. If possible, without putting yourself in
danger, get some shots of the vehicles in the positions where they came to
rest (before they are moved). Also, try to get some photos of any skid marks
that you can see.
Be sure not to get in the way of any emergency or law
enforcement personnel and don’t put yourself or others in harm’s way!
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Page
Article 3:
Accident Reconstruction Primer for
Attorneys
by Jack Debes, Ph.D.
and Ken Obenski, P.E.
Underlying Science
Accident
Reconstruction (AR) is based on the laws of
Newtonian physics.
Although many of us may have a tough time remembering these laws from
our high school physics class, we live in a world that is governed by these
principles so that even if we don’t remember the laws by name, we actually
have an intuitive feel for them.
Newton’s
First Law states that a body in motion
remains in motion unless acted upon by some outside force and a body at rest
remains at rest unless acted upon by some outside force.
Mathematically, the First Law
states that if the sum of all forces on a body is equal to zero, its
velocity is constant. In AR
this means that if a driver maintains a constant speed while traveling on a
level road in a given direction, the vehicle will continue at that speed and
direction unless the driver brakes, accelerates, turns, crashes into another
object, or the vehicle stops due to fuel consumption or mechanical failure.
Newton’s Second Law
states that Force equals Mass times Acceleration (F=ma). In AR, acceleration
is a measure of the rate of change of speed of the car. If it speeds up, the acceleration is positive, if it slows
down (decelerates) the acceleration is negative.
The force required to accelerate the car is equal to the mass of the
car times the acceleration. This
force, which is called the inertial force,
is what you feel when your body is pressed into the seat back when you put
the pedal to the metal; likewise it is the force that causes your briefcase
to fly forward off of the seat and onto the floor when you slam on the
brakes. When two vehicles
collide, each vehicle experiences a sudden change in speed called
acceleration (or deceleration). This
change in speed is known as Delta-V
or ΔV.
The change of speed divided by the time of the collision is equal to
the acceleration (ΔV/Δt=a). And
since Force is equal to Mass times Acceleration (F=ma), ΔV is related
to the force of the collision by the following relation:
F
= m a = m (ΔV/ Δt)
Or
solving for the same equation for ΔV, one can write:
ΔV=(F
Δt)/m
In
auto collisions, the time the vehicles are in contact and transferring
momentum (Δt) may vary, but typically it is about 1/10 second. Therefore, ΔV is generally a good measure of the
acceleration (a) and ultimately the force (F) of the collision.
Since force is what damages vehicles and injures occupants, ΔV
is generally a good way to quantify the severity of a collision.
Newton’s Third Law
states that every action has an equal and opposite reaction.
While this principal may also apply to human behavior, in physics it
means that if I lean against the wall with a certain amount of force, the
wall pushes back against me with an equal and opposite force.
In an auto collision the impacting car is referred to as the bullet
vehicle, whereas the impacted car is called
the target vehicle.
In an auto collision, the force that the bullet car applies to the
target car is equally opposed by the target car pushing back on the bullet
car.
Application of Principles
Accident
Reconstruction is based on the accurate science of Newtonian physics.
Unfortunately a scientific answer can be no more precise than the worst
input data, and input data is often assumed, guessed, estimated or crudely
measured.
One area that is confusing
is units of measure. Usually in
America speed S is expressed in miles per hour (mph) and velocity V
in feet per second (fps), but the terms may be used indiscriminately. In
physics V represents a vector, which defines both speed and
direction. However, in AR the
term V is often used for speed only with no direction specified.
A vehicle can leave tire
marks in at least three different ways:
1.
Acceleration Skid (“burn-out”)
2.
Braking Skid
3.
Centrifugal Skid (Yaw Mark)
Burn-outs tend to
start out dark and then get lighter as the vehicle gains traction.
Generally, speed cannot be determined from burn-out marks. Burn-out
skids can be curved or straight, and usually involve only one or two
tires (the drive wheels). Braking
skids on the other hand start out light and get darker as the skid
progresses, since the tire is heating up during the skid and laying down
progressively more rubber until the vehicle comes to rest.
Braking skids can involve from one to four tires (on a 4
–wheeled vehicle) and they are generally linear.
Centrifugal skids occur during turning and are therefore
curved.
The most basic
technique for estimating vehicle speed is by measuring the length of braking
skid marks. The pre-skid speed (mph) is estimated by first multiplying: 30
times the distance the vehicle skidded before stopping without
impact times the drag factor; and then taking the square
root.
30 is a derived mathematical constant. The distance a vehicle
skidded should be straightforward. It is simply the length of the longest
skid in most cases. If any other value is used, the reconstructionist should
explain why. Drag factor goes by many names, including mu, m,
F, C.O.F.
(Coefficient Of Friction). It is nearly impossible to know the exact drag
factor for a given accident because there are too many variables that
determine it. If the same
vehicle can be skid tested at the same location this will yield the best
available estimate. Tests of other vehicles or other locations may be
meaningful or worthless depending on how the test is done.
Fortunately, because the answer is based on the square root, small
errors in drag factor will be insignificant.
The weight of the vehicle is not needed.
If there are multiple
overlapping skids, it is necessary to inquire how the skid distance
was determined. There are many possibilities, but it is almost never valid
to add skid lengths together, or to take an average.
Often the vehicle does
not stop in one continuous skid. The vehicle behavior must be broken down
into separate regimes, then the speeds added together by taking the square
root of the sum of the squares.
There are different but perfectly valid ways to rearrange the same
mathematics so it need not look exactly as it does here.
It is seldom valid to simply add the individual speed estimates
together, because the sum is the sum of the energies, and energy is ½ times
mass times velocity squared (1/2 mv2).
Mass is an unfamiliar term to most people, but it is
intuitively similar to weight. Weight
is gravity dependent, mass is not. Your
weight on the Moon will be less than your weight on Earth.
Mass (measured in Slugs) times 32.2 equals weight (in pounds)
if you are on Earth. We have
not had an AR case on the Moon yet!
Another more
controversial method for estimating vehicle speed is the critical speed
for a curve based on centrifugal skids or yaw marks.
This calculation is
often used incorrectly. It can only be valid if the vehicle is in a steady
state condition, not out of control. Non-concentric
yaw marks imply the vehicle is going out of control.
At least three tires must leave marks that are concentric and
circular; otherwise the estimate must be adjusted for these non-standard
conditions. Valid application
of the calculation technique requires that the vehicle travel at a nearly
constant speed through the maneuver. This
will be evidenced by tire marks called striations that are radial
(i.e. perpendicular or nearly perpendicular to the direction of travel
indicated by the tire marks). Critical speed cannot be added to speed
calculated by other methods (such as speed from braking skids).
It stands alone as the speed of the vehicle at the time when the
concentric tire marks were made.
One of the most
difficult techniques for estimating speed is speed from crush damage.
The data are seldom adequate and the formulae alone take up most of a
page. The range of possible
speeds for an impact with a fixed object varies widely among vehicles.
However, assuming the vehicle is stopped by the impact, impact speed (mph)
is generally in the range of one to 1 ½ mph times the deepest crush
measured in inches plus the speed that the vehicle will tolerate
without being damaged.
Impact Speed
(mph) ≈ [(1 to 1.5) x Crush (in.)] + No Damage Speed
(mph)
In a front or rear
impact involving the bumper, bumper ratings can be useful in estimating the
no-damage speed. Speed
calculated from crush may be expressed as closing speed, barrier equivalent
speed, kinetic energy speed, crash speed, delta-V, DV
etc. These terms are not interchangeable, although in certain situations
some of these values can be equal.
Momentum
analysis can often lead to more accurate estimation of the speed when two or
more vehicles are involved. The
speeds after a collision can be estimated using the methods shown above
(i.e. skid distances and speed addition).
The pre-collision speeds can be determined from basic Newtonian
conservation of momentum. Mass
before collision times velocity before collision equals mass
after collision times velocity after collision.
M1b•V1b
+ M2b•V2b = M1a•V1a +
M2a•V2a
This can be simple if the
collision is in line, or very complicated if there is travel in different
directions before or after the collision. It will be complicated if there
are more than two objects involved, such as a motorcycle with people that
fly off in different directions. This
requires knowledge of the mass (or weight) of each involved object, as well
as estimates of the drag factor, distance and direction for each
post-collision movement plus an estimate of the pre-collision direction of
each object. The analysis can be done with massive vector equations
that can be very precise but prone to horrible error. Alternatively,
graphical techniques can be used which are easier to understand, but less
precise.
This document is a
brief description of the principles of vehicular Accident Reconstruction
(AR). The information
described herein should be adequate for attorneys, insurance adjusters, or
medical professionals to gain a basic understanding of the concepts and
vernacular of the art and science of AR without having to become an
expert in the field themselves. Modern
technology (e.g., Total Stations, Computer Simulation Software) developed
for AR data collection, analysis and presentation can simplify the
process of AR and provide
impressive graphics. However,
the validity of the analysis can only be as accurate as the raw data
collected. There is no
substitute for high quality photographs and accurate direct inspections of
vehicles and accident scenes.
Acknowledgement
The contributions of our colleagues L.L. Wickham,
Ph.D., E.S. Shapiro, A.S.E. and Lynda Laws are gratefully acknowledged.
Accident
Reconstruction Definitions
ABS (antilock braking system): Computer
intervention to interrupt skidding momentarily for improved control
simultaneous with maximum brake performance.
Acceleration: Rate of change in velocity with
respect to time.
Acceleration Skid: Tire mark left due to hard
acceleration, “burn-out” (informal).
Accident Reconstruction: The art and science of
using engineering principals to determine what really happened in an
anomalous event.
Accident: An unintended event with a negative
outcome.
Backlite: The
rear-facing window of a car, sometimes called the rear windshield.
Baker, J. Stannard: The pioneer of accident
reconstruction.
Barrier Equivalent Speed: The speed at which the
same vehicle would have to hit a fixed rigid barrier to sustain the same
magnitude of damage.
Bead: The reinforced part of a tire that engages
the rim.
Biomechanics: The application of engineering
mechanics to a biological system, especially the human body.
Bobtail: A short truck without a trailer,
specifically a truck tractor with no trailer.
Braking Skid: Tire
mark left due to hard braking.
Cab: The driver's compartment of a truck, not an
entire vehicle (see tractor).
Closing Speed: The speed of one vehicle relative
to another without regard to actual speed relative to the ground.
Crash Speed: An ambiguous term to be avoided.
Centrifugal Force:
Radial force acting on a vehicle while turning or negotiating a
curve.
Centrifugal Skids:
Tire marks left from a vehicle turning at critical speed (see Yaw
Marks).
Critical Speed: Speed at which the centrifugal
force of a vehicle negotiating a curve exceeds the traction force of the
tires on the road.
Crush: Permanent
damage or deformation sustained by a vehicle as a result of impact.
Deceleration: Negative acceleration.
Delta: Δ
(Greek), Difference or change.
Delta-V: (ΔV) Change in velocity, may be
cited in miles per hour (mph) or feet per second (fps).
Duals: Two tires at one wheel position.
Eighteen-wheeler: A maximum legal size truck,
usually having 18 tires on 5 axles.
g: Universal gravitational constant; for Earth,
32.2 feet per second per second.
GAWR: Gross axle weight rating, the maximum
total weight to be applied to the ground by that axle.
GVWR: Gross vehicle weight rating, the maximum
total weight the vehicle is designed to apply to the ground.
Human Factors: The study of the interaction of
humans with their working environment.
Hydraulic: Using moving fluid (liquid) that is
pressurized to do work.
Hydroplane: To be supported by the surface of
water by traveling at high speed.
Impact Speed: Closing speed at impact (see
closing speed).
Lay it down: To deliberately cause a moving
motorcycle to be upset onto its side.
OEM: Original Equipment Manufacturer.
Momentum: Mass or weight times velocity or speed.
Pressure: Force distributed over area.
Prolongation of a line: Poorly defined term used
in traffic accident reports. It
may be the true prolongation, a perpendicular offset to a point of tangency,
or a figment of someone’s imagination.
Quarter Panel: The sheet metal surrounding the rear
wheel area of a car or pickup.
Tractor: A truck built specially to pull a
semi-trailer.
Turning Circle: Diameter of the minimum circle
within which a vehicle can physically fit during a turn (a design
specification).
V: Generally designates Velocity.
Vector: A mathematical quantity having both a
magnitude and direction.
Velocity: Speed in a specified direction.
In accident reconstruction velocity is usually expressed in feet per
second and speed in miles per hour. Speed,
if expressed in feet per second, may be called (incorrectly) velocity.
Yaw mark: Tire mark made by a tire that is
side-slipping during a high-speed turn (see Centrifugal Skids and Critical
Speed).
Top of Page
Article 4:
Forensic Implications of Asphyxia
By Lori L. Wickham, Ph.D.
If you were to dive into a pool and hold your breath, how long could you
spend underwater? Probably less than a minute unless you are David Blaine,
an Ama Pearl Diver, or conditioned to lower your metabolic rate like a
meditating Yogi. However, with practice, many people can hold their
breath for about two minutes.
What do breath-hold diving, suffocation, strangulation, and drowning have in
common? They all involve progressive asphyxia, concomitant low
oxygen [hypoxia], high carbon dioxide [hypercapnia] and acidosis. The
latter is a result of the buildup of lactic acid, a by-product of anaerobic
metabolism [without oxygen]. Most of us are familiar with the feeling
of pain of lactic acidosis after a demanding workout. Asphyxia can be
limited to a regional tissue deprived of blood [e.g. ischemia]
or manifest as blocked respiration in the body as a whole.
There is a hierarchy within the body in terms of how long different
tissues can withstand this deprivation. In many cases, human
extremities can be deprived of blood for more than 30 minutes without damage
while the central nervous system, specifically those portions involved
in consciousness, will not continue to function for more than a few seconds
without oxygen. The disruption of cell metabolism in the tissues and the
accumulation of toxic by-products result in patho-physiological consequences
such as tissue necrosis, loss of consciousness and death. Forensic
interest may then become a question of causation and how long the asphyxia
lasted before death occurred. The latter may be important in cases where
family members witnessed the suffering of the decedent.
Lack of oxygen, either partial [hypoxia] or total [anoxia] causes death.
Normal room air is approximately 21% oxygen. Impairment
of cognitive and motor function can manifest at oxygen
concentrations of 10-15%, loss of consciousness at less than 10%, while
death usually occurs at less than 8%. For example, although hypoxic
endurance varies, a person can lose consciousness in 40 seconds and die
within a few minutes at ambient oxygen levels as low as 4-6%.
Asphyxial deaths, whether accidental, suicidal or homicidal are grouped by
forensic scientists into three generalized categories: Strangulation,
Chemical Asphyxia and Suffocation. Most reported murders by asphyxia
involve strangulation. An inhaled substance interfering with the body’s
ability to use oxygen [e.g. carbon monoxide, butane, and nitrous oxide]
characterizes chemical asphyxia. Carbon monoxide blocks the active binding
site of hemoglobin [Hb] the protein that carries oxygen in red blood cells.
‘Simple’ asphyxia is another term for oxygen displacement by another
gas. When water or another liquid fills the lungs causing asphyxia this is
called drowning. There are several variations such as near-drowning,
secondary drowning and immersion syndrome. Autoerotic or ‘sexual asphyxia’
by self-strangulation, drowning, choking, and a variety of other means is
increasingly reported, especially by the media.
Deaths due to suffocation are often subdivided further, by
causation, into those due to entrapment or environmental suffocation,
smothering, choking, mechanical asphyxia, mechanical asphyxia combined with
smothering and suffocating gases. Entrapment involves
individuals trapped in air-tight enclosures [e.g. children trapped in
abandoned refrigerators]. Environmental suffocation
usually involves someone accidentally entering an area depleted of oxygen by
a mechanism other than gaseous suffocation. For example, inhaled
fluid displaces air during drowning. Fungal infestations in enclosed areas
can also cause lethal oxygen reduction. Smotherings by mechanical
obstruction of the nose and mouth are rarely accidental. However,
examples are an alcoholic stupor leading to loss of consciousness and
subsequent face compression on pillows or bedclothes and defective cribs
with gaps that might impair infant mobility after laying face
down. Criminals also sometimes inadvertently asphyxiate a victim
with gags in the mouth or around the face. Choking asphyxia can be
homicidal, accidental or even a "natural" result of inflammation
of the soft mucosal tissues of the respiratory tract. Some find it
surprising that steam inhalation can result in swelling to the point of
obstruction in some reactive individuals. Most reported choking deaths
are accidental and involve obstruction of the pharynx and larynx by
food. Accidental inhalation of foreign material is another
cause.
When external pressure prevents breathing the term mechanical asphyxia is
used. Traumatic asphyxia, positional asphyxia and "riot-crush"
deaths are subtypes of mechanical asphyxia. In traumatic
asphyxia, a large mass or heavy weight presses on the victim's chest or
upper body preventing breathing. Survival with minimal or no pathology
is surprisingly common even if there is a short loss of consciousness.
More severe cases have included people pinned under a vehicle after a
motorway accident or a vehicle falling or rolling onto someone attempting
repairs. Traumatic asphyxia has also been reported during police restraint
when one or more officers attempt to subdue someone by sitting on their
chest. Positional asphyxia is most often an accidental result of someone
trapped in a restricted space in a position that prevents breathing.
Self-imposed suspended or strapped positions with the head lowered for
autoerotic benefits are examples of accidental or suicidal positional
asphyxia. Many cases involve alcohol or drug intoxication.
When someone is prevented from breathing or crushed by the bodies of others
such as during sports games and rock concerts, the asphyxial death is
termed "riot-crush" for obvious reasons. A combination of
traumatic asphyxia and smothering can be accidental or homicidal. The
former might involve inadvertently rolling over and incapacitating an infant
placed in bed with parents or an older sibling. Burial in a collapsed
mine-shaft or cave are other examples. During the early 19th
century, "resurrectionists" Burke and Hare excavated
graveyard bodies to sell to medical schools. They decided
preying upon alcoholics would make their job easier. Burke sat on
the victim's chest, used one hand to cover their nose and mouth and the
other to close their jaws resulting in traumatic asphyxia and a fresh body -
without digging. This is an example of homicidal traumatic asphyxia in
combination with smothering now called "Burking".
During deaths by suffocating gases, ambient oxygen is displaced by
another gas. Common examples are carbon dioxide and methane
which are found in mines, sewers and natural gas used for cooking. An
interesting case that has received media attention recently involves the
mass mortality of people living near volcanoes [www.pbs.org/wgbh/nova/volcanocity].
Lake Nyos and Lake Kivu formed in craters made by dormant volcanoes in
Africa. Fissures or springs beneath the craters feed the lakes and deliver
high concentrations of carbon dioxide gas which can flow beneath the ambient
air to lower elevations thereby decimating entire villages including
thousands of people and animals. The locals call these ground hugging plumes
of odorless, volcanic carbon dioxide "mazuku" which means
"evil wind". Children are often victims because they breathe air
nearest to the surface of the earth.
Tolerance to ischemia and asphyxia vary with not only age and special
adaptation but also with past medical history and present state of
health. For example, those who have a history of cardiovascular or
pulmonary disease may be more susceptible [e.g. heart attack, asthma].
Medications can also affect the body's ability to defend itself against
asphyxial threat.
Postmortem examinations, review of medical records, accident reports
and photos taken at the scene are used to analyze and classify asphyxial
deaths. There are non-specific physical signs used to attribute death
to asphyxia. These include visceral congestion via dilation
of the venous blood vessels and blood stasis, petechiae, cyanosis
and fluidity of the blood. Petechiae are tiny hemorrhages. Blood
vessels, usually small veins, are broken by high intravascular
pressure. They can occur in various parts of the body such as over the
surface of the heart and organs, in the eye, the skin and the scalp.
If a large area is affected they may be termed ecchymoses and appear as
bruising. Hemoglobin [Hb] in red blood cells turns from
red to blue when it loses oxygen. Veins are described as blue since they
carry blood that has released oxygen to the body’s cells, back to the
lungs where it can be reoxygenated. As asphyxia progresses and more oxygen
is depleted, a dark discoloration of the skin and tissues called cyanosis
develops. Cyanotic tissue is described as blue, black or purplish in color
depending upon the percentage of deoxygenated blood. After death, changes in
blood chemistry and the breakdown of clotting proteins such as fibrin lower
the viscosity of the blood; this is sometimes called 'fluidity'. The study
of flow is called rheology thus; those who specialize in the study of blood
flow behavior are called rheologists or, more specifically, hemorheologists.
As stated earlier, these physical variables are non-specific to asphyxia
meaning they can be present after death from other causes.
Furthermore, a case may be complicated by pathology or injuries additional
to asphyxia. This information is used by forensic scientists in combination
with data on place and manner of death to perform analyses and form
opinions. Investigation into asphyxial death often involves a combination of
experts who may be clinicians, biomechanical experts and automotive
experts who perform accident reconstruction, and chemical or
biological scientists.
Further Reading
Beach, H.H. A. and F. Cobb. 1904. Traumatic Asphyxia. Report of a recent
case, with a study of the minute pathology, and summary of reported cases. Annals
of Surgery 36(4): 481-494.
Feldman, E.A. 1969. Traumatic Asphyxia. The Journal of Trauma, 9(4):
347-353.
Elsner, R. and B. Gooden. 1983. Diving and asphyxia: A comparative study
of animals and man. Monographs of the Physiological Society, No. 40.
Cambridge University Press, New York, NY, 168 pages.
Elsner, R. and L.L. Wickham. 1988. Implications of physiological studies
of seals. Marine Mammal Science, 4(1): 34-43.
Wickham, L.L., R.M. Bauersachs, R.B. Wenby, S. Sowemimo-Coker, H.J.
Meiselman, and R. Elsner. 1990. Red cell aggregation and viscoelasticity of
blood from seals, swine and man. Biorheology, 27: 191-204.
Hambeck, W. and K. Pueschel. 1981. Death by Railway Accident: Incidence
of traumatic asphyxia. The Journal of Trauma, 21(1): 28-51.
Copeland, A. R. 1986. Vehicular-related traumatic asphyxial deaths – Caveat
Scrutator. Z. Rechtsmed, 96: 17-22.
DiMaio, V.J. and D. DiMaio. 2001. Asphyxia. In Forensic Pathology,
2nd Ed., New York, NY, pp. 229-275.
Adams, V.I. and R.S. Vega. 2004. Suffocation in Motor Vehicle Crashes. The
American Journal of Forensic Medicine and Pathology, 25(2): 101-107.
Miyashi, S., K. Yoshitome, Y. Yamamoto, T. Naka and H. Ishizu. 2004.
Negligent homicide by traumatic asphyxia. International Journal of Legal
Medicine, 118: 106-110.
Sauvageau, A. and S. Raclette. 2006. Autoerotic deaths in the literature
from 1954 to 2004: A review. Journal of Forensic Science, 51(1):
140-146.
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of Page
John Fiske Brown Associates, Inc. (2008)
Forensic Engineering, 
In a perfect world, the vehicle would be in "as
manufactured" condition in all cases and we could simply blame the driver
for the collision. Not necessarily. Numerous vehicles suffer from design or
manufacturing defects right off the assembly line. Scroll through a list of your
favorite car’s TSB’s (Technical Service Bulletins) and you’ll see what I
mean. Vehicle manufacturers issue these notifications to service personnel when
problems exist. Manufacturers are well-meaning, but even a car in good condition
can be the cause of an accident because of production or design deficiencies. A
few examples are: