Technical paper addressing impact and collision forces incorporated into masters degree program dealing with biomechanical trauma for physicians conducted by Lynn University and the University of Miami Medical Center.
N - Motor Vehicle Accident Reconstruciton and Biomechanical Physics
Motor Vehicle Accident Reconstruction
& Biomechanical Physics
Robert C. McElroy, Ph.D.
www . ForensicAccident . Com
Accident reconstructionists rely on a wide range of methods to record
and analyze motor vehicle accident information. This paper addresses con-
temporary methods of obtaining and analyzing collisions with emphasis on G
force explanation for biomechanical analysis.
Traffic accident reconstruction is the effort to determine, from whatever
resources are available, how an accident happened. A traffic accident
reconstructionist must be familiar with the application of a wide range of
mathematics and specialized aspects of vehicle technology. Because of the
wide range of knowledge required by the accident reconstructionist, voluntary
certification is available through the Accreditation Commission for Traffic Ac-
cident Reconstruction. ACTAR certification includes education, work experi-
ence, and successful completion of a comprehensive examination.
Mathematics are at the core of traffic accident reconstruction. Many
different equations are used to determine different aspects of an accident. Sir
Isaac Newton developed three mathematical laws of motion which provide the
foundation for traffic accident reconstruction.
In Newton’first law of motion an important property of matter ap-
pears. It is known as inertia, that property of matter by which an object main-
tains a constant velocity in the absence of an unbalanced external force. When
an automobile is suddenly stopped, the passengers obey Newton’first law
and continue in their motion with constant velocity until some external
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 1
force changes their state of motion. Seat belts in a automobile can provide
such an external force which is much preferred to that exerted by the wind-
shield or dashboard. Another statement is the following:
A body at rest remains at rest, and a body in motion remains in motion with
constant velocity along the same straight line unless acted upon by some outside
Newton’First Law of Motion: Inertia of Rest & Motion
Newton’ first law states that a body at rest
s Newton’ first law also states that a body in
remains at rest unless acted upon by some motion remains in motion with constant velocity
external unbalanced force. unless acted upon by some resultant force.
Newton’second law states that if a body is acted upon by an unbal-
anced force F, its center of mass will accelerate in the direction of the force.
The acceleration, a, is proportional to the force, F, and the constant of propor-
tionality, m, is called the mass of the body. Another statement is the following:
The acceleration of a body is directly proportional to the resultant force action upon
the body and acceleration is inversely proportional to the mass of the body.
Newton's second law provides the key relationship between force and
acceleration since force and acceleration are vectors and vectors have both
magnitude and direction. Mass is only a magnitude, so it follows that the
magnitude of force equals the magnitude of acceleration times the mass.
Unification of these concepts reveals that force direction must be the same as
the direction of the acceleration because mass does not have a directional
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 2
The second law is written F = ma where the unit of force is the new-
ton. One newton produces an acceleration of one meter per second, per
second, in a mass of one kilogram. One newton has a value of .2248 lb.
Newton’Second Law of Motion: F = m a
Acceleration of a body is directly Acceleration due to a given result-
proportional to the resultant force ant force is inversely proportional to
acting on the body. the mass accelerated.
Newton's third law is equally valid in dealing with bodies at rest or
in motion, either uniform or accelerated. The wheels of an automobile in
motion push backward on the road, but the road pushes forward on the
wheels with an equal force during acceleration. Another statement is the
Whenever one body exerts a force upon a second body, the second body exerts an
equal and opposite force on the first.
Newton’Third Law of Motion: Action = Reaction
Action (force exerted on the trailer) is equal to the
reaction (force exerted on the car by the trailer).
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 3
Prediction of what happened during a collision by examination of what
remains in the form of residual damage can be used to calculate speed change
or Delta Velocity (DV) experienced by the vehicles in the collision. DV is one
of the best available measures of accident severity.
DV assumes that collision stopping force on a vehicle is a linear function
of residual crush depth. Up to a certain force level, there is no permanent
damage and beyond that point, permanent damage increases with increased
force. Two stiffness coefficients, A and B, define the force-damage curve.
Fundamental to a solution for speed change are appropriate A and B values
for a specific vehicle. A and B values are derived from the collision damage
sustained from known velocity changes of a vehicle into a barrier. Therefore,
A and B values can be used to calculate speed change based on permanent
vehicle crush (see chart).
Vehicle tests sponsored by the
National Highway Traffic Safety
Administration resulted in a series of
computer programs released to the
public called CRASH. The last public
version CRASH3 was revised and
released in 1982. Several computer
based accident analysis programs are
available for the accident
reconstructionist, each ultimately
stems from this background.
A collision analysis project is
defined as a series of step by step
calculations. The investigator will organize the project into separate events
which require solutions for specific unknowns. Normally, each event or
step will consist of an equation with only one unknown. The art of collision
analysis is to determine which event to solve first and then how to proceed
with the next calculation to develop a unified collision sequence analysis.
Calculations from a popular AI Tools computer program help to address
specific information needed in order to piece together the accident
reconstruction. When properly used each calculation will help to fill in a missing
piece of information. Here is a list of AI Tools calculations:
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 4
Eq Group Eq # Problem Description Special 45 Tangent Offset
Speed 1 Speed from Distance & Drag 46 Radius from Chord and Mid-Ordinate
2 Constant Speed from Distance & Time 47 Critical Curve Speed
3 Speed from Drag & Time 48 Combined Speed from Drag Surfaces
4 Final Speed from Start Speed, Drag & Time 49 Combined Speeds
5 Final Speed from Start Speed, Drag & Distance
6 Start Speed from Final Speed, Drag & Time Airborne 50 Horizontal Launch and Fall Speed
7 Start Speed from Final Speed, Drag & Distance 51 Small Angle Speed at Launch Equation
52 General Projectile Speed Equation
Time 10 Time from Constant Speed & Distance
11 Time from Speed & Drag Energy 60 Speed from Linear Kinetic Energy
12 Time from both Speeds & Drag 61 Kinetic Energy from Speed and Weight
13 Time from Distance & Drag 62 Force from Kinetic Energy and Distance
63 Distance from Kinetic Energy and Force
Distance 20 Distance from Speed & Drag
21 Distance from Constant Speed & Time Motorcycle 70 Lateral Acceleration Factor from Speed and Radius
22 Distance from Two Speeds & Drag 71 M/C Lean angle from Speed and Radius
23 Distance from Initial Speed, Drag & Time 72 Turning Radius From Speed and lateral acc
Acceleration 30 Drag from Speed & Distance Animation 80 Final Speed from Start Speed, Drag and Time
Factor 31 Drag from Speed & Time Assist 81 Distance from Start Distance, Speed, Drag and Time
32 Drag from both Speeds & Time
33 Drag from both Speeds & Distance Reaction 90 Total D from P/R time, Speeds & Drag f
34 Drag from Distance & Time Times 91 Total D from Start Speed, Times & f
35 Drag from Initial Speed, Distance & Time 92 P/R Time from Speeds, Total D & f
36 Drag from Road Friction, Brake Efficiency & Grade 93 P/R Time from Start Speed, Total D, f & Time
37 Drag from Horizontal Force & Weight 94 Start Speed from Total D, Times & Drag
95 Start Speed from Total D, Final Speed, P/R Time & f
Linear 40 General Two-Dimensional Momentum 96 Drag from Speeds, P/R Time & Total D
Momentum 41 Inline - V1 from V2, V3, and V4 97 Drag from Start Speed, Total D & Times
42 Inline - Coefficient of Restitution 98 Total Time from P/R Time, Speeds & f
43 Inline - Plastic, V3 = V4 99 Total Time from P/R Time, Brake D & Drag
44 Inline -Elastic
Equation #1 Speed from Distance &
Drag. Calculation of vehicle speed made
from skid distance and drag factor.
Measured skid distance is 120 feet and
deceleration factor for the vehicle &
road surface combination is .7 G. The
vehicle is calculated to have been going
50 mph when the brakes were applied.
Momentum is a restatement of Newton's laws in a form that is useful for
the collision analysis or any event which involves very short periods of time.
The second law for a single object would be rewritten as F Dt = m DV.
The left side of the equation is the Impulse and the right side is the Change in
Momentum. This relationship is still a vector relationship. Force has the
same direction as the change in momentum which has the same direction as
the change in velocity. In summary:
Impulse = Change in Momentum
If two vehicles collide, the force or impulse on one of the vehicles is equal
and opposite to the force on the other. This is a consequence of Newton's third
law. Changes in momentum for both collision vehicles cancel. There is no gain
or no loss of momentum during a collision. Momentum before the collision
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 5
equals the momentum after the collision and because weight is propor-
tional to mass the final equation can be rewritten as:
W1V1 + W2V2 = W1V3 + W2V4.
On the left is a three car collision. AI
Tools Linear Momentum module below,
calculates that the Chevy Lumina came
into the collision at 27 mph and that the
Dodge had an entry speed of 15 mph.
Equation #1 was initially used to deter-
mine slide to stop, or departure, speeds
for each vehicle.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 6
Medical personnel involved in an accident investigation can provide
valuable injury information that assists in cause analysis. Where possible in
fatality accidents, autopsies should be
performed to determine the cause of death
and record information about the injuries.
Nonfatal injury information is also useful
to the investigator. Location of broken
bones is especially useful when
graphically represented in a skeleton
diagram in a seated position, on left. Injury
illustration could be done for each
occupant to produce a composite occupant
diagram for the vehicle. Injuries will
indicate the direction of crash loading for
the vehicle. Location of bruises and
contusions can also be addressed, since
Injury location identifica-
tion from bone fractures
these injuries can sometimes indicate use
in illustration. or nonuse of a seat belt or shoulder
Head injuries are important clues. If an instrument panel, roof pillar,
steering wheel or glass has evidence of a head strike (i.e., blood, skin, hair,
dent, teeth), that spot should be documented. With a specific location for the
strike and the relationship to the occupant’body, the investigator can evalu-
ate the angle of head impact, body position, and restraint system function, see
below. It is important to remember that an occupants head motion is exactly
opposite to the crash loading direction.
Correlation of head strike information between passengers and vehicle’interior.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 7
In a biomechanical investigation approach, the most important task is
to determine occupant crash loads and probability of serious injury. Other
investigative tasks help the biomechanical investigator to understand physi-
cal relationships in the accident which lead up to either receiving a serious
injury or not. To calculate an average G force crash pulse, the following equa-
tion is used for each axis of occupant travel.
In this equation G = the average force on the specific occupant and is
expressed as a multiple of occupant weight. Because of crash dynamics, peak
G figures will typically be twice the average G.
V = velocity change at the major impact, expressed in ft/s
g = acceleration of gravity, 32.2 ft/s2
S = deceleration distance, expressed in ft
The biomechanical investigator should look for clues in each axis (i.e.,
roll, pitch, yaw) for velocity change and stopping or deceleration distance to be
able to determine a unique crash loading for each occupant.
An extreme example of how the different G loading is experienced by
different people decelerating over different distances is found in this illustra-
tion. To understand the concept, consider a long uniform airplane fuselage
that crashes head-on into a cliff at 200
mph with Persons A, B, and C who decel-
erate in the crash distances of 5 ft, 20 ft,
and 45 ft, respectively. A calculation of
the average G-loading experience by Per-
sons A, B, and C would be 266G, 66G, and
29G, respectively. The average G calcu-
lation for passenger C is:
The average forward load on Persons
A, B, and C can be calculated by multi-
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 8
plying each persons weight by each persons average G load. If the weights of
Persons A, B, and C were 200 lb, 170 lb, and 100 lb, respectively, the average
loading experienced would be 53,395 lbf, 11,346 lbf, and 2,966 lbf respectively.
The weight load calculation for passenger C would be:
This airplane example assumed seats and restraints that held through-
out the crash sequence. For such a severe crash, it is not likely for all of the
seats and restraints to remain attached. A seat will fail when its maximum
load capability is exceeded. Assume seats with integral shoulder harness and
lap belts were designed for 25G static loads in the forward direction. Person
C of our airplane example would have a seat where the minimum force before
seat separation can be expected of 25 times their weight of 170 lb x 25G or
The load experienced by Person C was 2,966 lbf average or 5,932 lbf
peak. Thus, the seat for Person C would separate when its peak load exceeded
the design load capability of the seat. The more damaging loads for Person C
will occur later, when he or she undergoes major deceleration.
A vehicle traveling 35 mph sustained two feet of uniform crush. Substi-
tution into the formula reveals the G forces for this accident.
Gavg = = = 9.5 Gavg x 2 = 19 Gpeak
2gS 2 x 32.2 x 2
Federal Motor Vehicle Safety Standard, FMVSS
207 Seating Systems. Under this standard the seat
must be capable of withstanding a force “ times the
weight of the seat applied in a forward (S4.2.a) or rear-
ward (S4.2.b) longitudinal direction.” Thus if the seat
weighs 30 lb then it must not fail at a level below 600 lb
when calculated as 20 x 30 lb = 600 lb. It is important to
note that occupant weight is not considered. S22.214.171.124
Static force specifies 20 times the weight of the seat back.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 9
VERTICAL CRASH ANALYSIS
A more detailed analysis will reveal that the G loading will change
throughout the crash sequence. For example, crash loads experienced by an
occupant of an extremely high vertical velocity impact are shown on page 10.
From point A to B, the crash loads are low (typically 2 to 3 G) as the landing
gear deforms. Point B is where the fuselage lower skin contacts the terrain.
Loading from point B to C to D to E is extremely high as the aircraft floor
comes to rest at point F.
If an occupant is sitting on the floor, the loading experienced would have
been points B to C to D to E to F. Note the horizontal line, which is an injury
load threshold (within time duration limits) above which severe injury is ex-
pected. If the occupant is sitting on a seat, the vertical crash loading experi-
enced will rise from point B to C and then drop to point G. Loads do not exceed
point C, as this is the maximum strength of the seat which fails at Point C.
The occupant’load is about zero from point G to H because the occupant is
basically free-falling from a seated position until contacting the floor at point
Vertical G loading during a vertical
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 10
However, the aircraft floor is just about to come to rest at point F. Thus,
the occupant impacts a nearly stationary floor and the crash loads experi-
enced by the occupant will go from point H to I to J. Unfortunately, occupant
load penetration above the injury zone threshold line would indicate that a
severe injury would be expected for the example occupant. It is obvious that
an average G loading for an entire aircraft is inaccurate and misleading.
Understanding the crash loads on an occupant is not possible without good
information from the crash survival investigation on injuries, restraints, seat
damage, and fuselage damage.
AERIAL CRUSH PHOTOGRAPHY
On-site aerial photographs taken with the aid of a boom and perimeter
grid capture collision data. This system has proven to be extremely useful in
addressing collision dynamics. A brief synopsis of this method reveals its use-
fulness to the accident reconstructionist and biomechanical expert.
Securely place the base against a tire.
Elevate & lower the boom gently.
Front View of Camera & Grid
Top View of Camera & Grid
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 11
At night a westbound pickup
truck was towing a 2700 pound air
compressor. The air compressor dis-
connected from the pickup, crossed
the centerline of a two lane roadway
and struck an eastbound 3,100
pound automobile. The speedometer
of the car was “crushed” and read 55
mph. Delta V calculated at 108 mph.
Below is a composite photogrammet-
ric assembly for this collision. Aerial
photographs of the car and air compressor were placed together in order to
illustrate maximum engagement at collision. A perimeter grid has been placed
around the car. Black markings one foot apart are used for post collision
analysis by permitting lines to be drawn across the photograph for documen-
tation of crush sustained by the vehicle.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 12
VEHICLE CRUSH DAMAGE
Analysis of photographs reveals that the subject vehicle sustained 6.3
feet of crush as a result of the collision.
Exemplar Vehicle With Pre Crash and Maximum Engagement
Driver to Bumper & Air Compressor Illustration
Subject Vehicle With Roof Folded Back
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 13
VEHICLE CRUSH DAMAGE - SIDE VIEW
These CAD illustrations
show the vehicle side view
before and after impact and
the relative position of the
driver inside the automobile.
Post-collision vehicle defor-
mation at right illustrates the
vehicle contact with the
driver. It can be clearly seen
that this collision was not
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 14
Linear momentum calculations show a closing speed for this head-on
collision of approximately 108 mph and Equivalent Barrier Speed (or EBS) of
59 mph. EBS is when a vehicle impacts a massive barrier which absorbs no
energy of collision. It is a convenient concept to compare the energy absorbed
in crushing vehicles. The National Highway Traffic Safety Administration
annually releases its New Car Assessment Program (NCAP) crash test results
for current model year vehicles. These tests give occupant injury criterion
values for collisions at the 35 mph EBS. In the EBS crash of an exemplar Buick
Century, approximately 2' of uniform crush was sustained.
As previously addressed, an increase in
velocity at impact results in an increase in the
energy of collision. The subject vehicle had an
EBS of 59 mph which results in an increase of
the energy involved in this collision of 184%, or
almost three times as much energy being pro-
duced, as in the 35 mph crash tests. This
massive increase in energy is translated di-
rectly into an increase in the forces making
this collision unsurvivable.
The air compressor has an overall width of 56 inches, 13" less than the
Buick, and weighed almost as much. It approached from the driver's side at
a slight angle of about 8°. The net result of these factors was primary damage
and collision force concentration in an area directly in front of the driver. Force
concentration is shown in illustrations included in this report. The previous
page however, best illustrates the total amount of intrusion into the driver's
side area. Measurements are shown of the distance from the driver to the left
front of the vehicle both before and after collision. These measurements
reveal that the initial 9.2 ft. of vehicle in front of the driver had been crushed
to 2.9 feet. Thus, the area immediately in front of the driver sustained a total
crush of approximately 6.3 feet.
Extremely high EBS loads coupled with massive collision damage con-
centrated directly in front of the driver, made this collision unsurvivable under
any circumstances by the driver of the Buick.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 15
LOW SPEED COLLISIONS
According to General Motors, in 1986, more than 33% of all automobile
injuries occurred in low-speed collisions where the speed difference between
the vehicles was less than 20 mph. It has also been estimated that more than
75% of low-speed collisions resulting in injury are rear-end collisions.
Over the past forty years, there has been a significant amount of research
into the effects of vehicle collisions and resultant occupant movement. However,
the majority of this research concentrates on the effects of high collision speeds,
typically 30 mph or above. Most staged collisions performed by car
manufacturers, insurance institutions (Insurance Institute for Highway
Safety), and the United States Government (National Highway Traffic Safety
Administration) are frontal collisions at speeds of 30 to 35 mph into a rigid
The four main types of low-speed collisions are 1) rear-end, 2) front-end,
3) lateral, and 4) side-swipe.
Rear End Side
Front End Lateral Swipe
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 16
In many low-speed collisions, occupants claim common “ whiplash”
symptoms such as pain in the neck, shoulders, arms, and low back, despite the
absence of vehicle damage. Investigators are frequently asked whether the
claimed injuries could have resulted from what appears to be a trivial event.
To address this question, investigators must first determine the severity of
the collision and then compare that value to human injury tolerance levels.
Before looking at the ways to determine the severity of a low-speed impact,
it is first important to understand the difference between high-speed and low-
speed impacts. When two vehicles collide at a high speed, they essentially act
like two balls of clay. They deform on impact and remain deformed with little
or no crush energy being released after impact. There is little or no “ bounce”
or elastic behavior during the impact. The engineering term for this “ bounce”
or elastic behavior is restitution. In high-speed collisions, the restitution
approaches a minimum value of zero and, therefore, when reconstructing high
speed collisions, restitution is usually ignored.
When two vehicles collide at a very low speed, they act more like two
tennis balls. A large percentage of the deformation of the vehicles is elastic in
nature and is released after impact. Only a small percentage remains as
permanent crush damage. In low-speed collisions, the restitution approaches
a maximum value of one. Therefore, when reconstructing low-speed collisions,
the effects of restitution cannot be ignored.
The severity of a collision is quantified by the acceleration (or
deceleration) experienced by a vehicle during impact. Acceleration is speed
change divided by time. If collision duration is assumed essentially constant,
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 17
then velocity change can be used to quantify collision severity. This is a valid
assumption for most collisions where the time duration is approximately 1/
10th of a second. However, in collisions such as underride impacts, the
In general terms, severity of an impact is related to vehicle damage. A
vehicle that has sustained several inches of rear-end crush has experienced a
more severe impact than the same vehicle that has less or even no permanent
rear-end damage. However, there are significant differences between the
relative strengths of different surfaces of the same car, and between the same
surfaces of different cars. For example, the rear ends of two different cars will
not be equally strong, so that two different cars with similar damage may not
have experienced the same impact severity.
Where there is bumper engagement with no damage in a rear or front
impact, it is often possible to determine impact severity from an inspection of
the vehicle bumpers. In many cases, the amount of compression of bumper
isolators can be correlated to the vehicle’ V (velocity change) in a minor
front or rear impact. In non-isolator equipped cars, which are increasingly
more common, the task of determining severity is more difficult. For many
passenger cars in North America, there is often little or no damage after a
minor impact. Some Asian cars in particular have quite robust foam-core
bumpers that are more damage resistant. Attached is a list of some vehicles
equipped with foam core bumpers.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 18
In lateral and side swipe impacts, damage is more noticeable, since body
panels, which are much weaker than bumpers, are involved. Body panels dent
and horizontal scuffs and creases are easily produced with very minor impacts.
Currently in the United States, a Federal safety standard requires that
automobile bumpers keep damage away from car bodies in 2½ mph front and
rear into flat barrier impacts. Damage is allowed to the bumper itself. These
requirements are much weaker than the stronger 5 mph no-damage bumper
rule that was in effect during the 1980 to 1982 model years. Neither strong
nor weak bumper requirements have ever applied to trucks or vans.
Low speed crash tests indicate that many bumpers are built to exceed
the standard and in some cases are undamaged at speeds well in excess of
those set out in the standards.
DAMAGE & SEVERITY ASSESSMENT
Damage sustained by vehicles during impacts varies greatly among
models and manufacturers. Certain vehicles show no evidence that an impact
occurred, even after impacts with severities as high as 10 mph V while others
have incurred structural damage during very low severity impacts. These
differences in vehicle behavior help explain instances where one vehicle will
show large amounts of deformation while the other vehicle will appear
Often, investigators will underestimate the impact severity because no
damage was observed during the vehicle examinations. Similarly,
overestimates have also been made when the vehicles show obvious damage.
Closer investigation into vehicles’ properties will provide insight into
relationships between vehicle damage and the corresponding impact severity.
Results from staged human volunteer collisions is significant. From this
data, it can be seen that the type of vehicle engagement during collision and
type of impact both need to be considered when determining collision severity.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 19
It should be noted that the severity given in the table represents the
probable maximum severity. This maximum value is only used if there exists
no test data for the same or similar model vehicle, or any other evidence, to
indicate a lower damage threshold.
Collision severity can also be determined from physical evidence at the
scene of an accident, such as vehicle final rest positions and pre-impact
acceleration distances, skid marks, etc. Unfortunately, this physical evidence
will typically not be recorded by police because the accident is viewed as a
Most computer programs used to determine speed from crush damage
(eg. SLAM, EDCRASH) are written for high-speed collisions and assume a
linear (straight line) relationship between crush and speed. They use vehicle
crush characteristics, or stiffness coefficients, obtained from crash tests typically
performed in the 30 to 40 mph range. In this speed range, for most vehicles,
there is essentially a linear (straight line) relationship between crush and
speed. Consequently, these computer programs give good results for barrier
equivalent speeds of 20 to 50 mph.
When these computer programs are used to analyze low speed collisions
the same linear relationship is assumed to exist at low speeds. The results
obtained from 30 to 40 mph staged collisions are extrapolated backwards.
Results from staged low speed collisions indicate that this assumption is
incorrect. For most vehicles there is not a linear relationship between speed
and crush at low speeds, note the dashed lines above. Therefore, these computer
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 20
programs should not be used to analyze low speed collisions unless great care
is used to modify the vehicle crush coefficients.
HUMAN INJURY TOLERANCE LEVELS
When trying to understand the motion of an occupant subjected to a low
speed front- or rear-end collision it is useful to visualize the occupant as a
simple head-on-a-stick. Depicted below is a rear-end collision.
R o t a t io n
Ar m A c c e l e r a t io n
Whenever a vehicle is rear-ended, everything moves forward. This in-
cludes the vehicle, the seat, the occupant’torso and the occupant’ head.
However, there is differential motion. The car and seat move fastest, the torso
initially moves more slowly, and the head moves slowest of all. Consequently,
the occupant suffers the sensation of “sinking back” into the seat, while their
head suffers a rearward rotation (this is the origin of the so-called “whiplash”
mechanism). As the torso sinks back into the seat (but actually moving for-
ward), the seat compresses like a spring. Ultimately, this compression stops
and the torso reaches the same speed as the car.
Thereafter, the seat “
unloads” and acts as a damped spring, resulting in
the torso moving forward faster than the car. As a result of this “ bounce,”
the torso can move forward up to about 1.3 times the speed change that the car
experiences due to impact. Thus, if the car experiences a velocity change of 5
mph due to a rear impact, the torso can end up moving at up to 6.5 mph.
Whether the occupant actually experiences this increase in speed depends on
the lockup behavior of the seat belt, especially the shoulder belt.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 21
This composite illustrates the motion of an occupant during a rear-end
impact. A potential for injury occurs when the head is fully rotated backwards
, known as hyperextension, or fully rotated forward , known as
hyperflexion. A properly positioned head rest can reduce the amount of
backward head rotation and hence reduce the potential for head injury.
Once the severity of the collision has been determined, then this value
can be taken and compared to human injury tolerance levels. Results from
daily activities or volunteer exposure to staged low-speed collisions can be
1. Daily Activities: The loads that an occupant was subjected to during
a collision can only be compared to daily activities if the direction, duration,
and magnitude of the loads are the same. For short time durations, less than
a second, human injury tolerance is sensitive to the time duration that the
load is applied. A higher load can be applied without injury if the time duration
If an individual jumps off of a table on to a solid floor, then he or she will
be subjected to a fairly high deceleration on impact. But injuries typically do
not occur because time duration of the impact is of the order of a 1000th of a
Tests carried out with amusement park bumper cars reveal that, during
collisions, vehicle velocity changes as high as 5 m.p.h. can occur. The time
interval of these impacts is comparable to the typical duration of a low speed
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 22
2. Volunteer Exposure: Low-speed collisions with human volunteers
have been quantified in the chart below.
It should be noted that in the staged rear-end and lateral collisions,
none of the reported symptoms lasted longer than three days. Several
interesting trends can be observed from their results. In the staged rear end
collisions, symptoms started to be reported at collision severities in the 4 to 5
mph range. In the frontal and lateral staged collisions, symptoms started to
be reported at about 10 mph. Results indicate that the injury threshold level
for a frontal collision is greater than for a rear end collision. This is consistent
with typical real life low speed rear-end collisions where occupants of the struck
vehicle report injuries, but the occupants of the striking vehicle do not.
When using results from volunteer exposure tests, an investigator should
be aware of the following limitations:
1. Most of the staged collisions are bumper to bumper impact and not
2. Most of the volunteers have been male.
3. Most of the volunteers face forward at the time of impact.
4. Most of the volunteers are under 50 years of age.
5. Most of the volunteers are seat belted.
6. Most of the volunteers do not have preexisting conditions.
7. Most of the volunteers are mentally prepared for the impact.
8. It is difficult but not impossible to insure that a volunteer is
subjected to an unexpected impact.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 23
The table below combines MacInnis Engineering's vehicle damage
threshold and volunteer exposure results. From the table it can be seen that
in a rear-end impact it is possible for occupants to sustain symptoms when the
vehicle has no visible damage.
SUMMARY AND CONCLUSION
Use of results from staged low speed collisions, conducted at known
speeds, it is possible to determine collision severity of a real life low speed
collision. The determined impact severity can then be compared to results
from staged low speed collisions to determine if an injury threshold has been
Volunteer exposure tests give a good indication of the collision severity
at which symptoms, typically lasting less than 2 to 3 days, start to appear (i.e.
the injury threshold). Because human volunteers are used, the severity of the
staged collisions is typically not increased beyond the injury threshold severity.
Therefore, there is currently little or no data to indicate the relationship between
injury severity and collision severity above the injury threshold.
Accident reconstruction and biomechanical analysis with their respective
mathematical interpretations provide the basis to assess vehicle damage and
calculate injury loads sustained by the occupants.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 24
Vehicle damage photographs taken at ground level and with the aid of
an aerial boom are important in biomechanical analysis and accident
reconstruction. These photographs permit detailed analysis of crush and
principle direction of force. Post production photographic techniques can be
used to compare the damaged subject vehicle with an undamaged exemplar
An accident reconstructionist and biomechanical investigator should keep
in mind their job is to record facts necessary to mathematically analyze an
accident. Some data or clues are perishable and must be obtained or
photographed early. Each occupant has a unique level of injury and experiences
unique crash loads depending on their location and other accident factors.
The purpose of the biomechanical analysis is to determine means and rationale
for injuries and improve the chances of survival in future collisions.
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tute of Police Technology and Management, Jacksonville, FL, 1988.
2. Boddorff, Thomas C. and Ian S. Jones, “Simple Overhead Photography
Techniques for Vehicle Accident Reconstruction”S.A.E. Paper No. 900370,
Society of Automotive Engineers, Warrendale, PA, 1991.
3. Baker, J.S., Traffic Accident Investigation Manual, Northwestern
University, Evanston, IL, 1975.
4. AI Tools, AR Software. Trantech Corporation, Redmond Washington.
5. McElroy, Robert C., “
Aerial Crush Photography & Analysis For Acci-
dent Reconstruction”Special Problems in Traffic Accident Reconstruction,
IPTM, University of North Florida, 1994.
6. Fox, Roy G., “Helicopter Crash Survival Investigation”Proceedings of
23rd International Seminar of the International Society of Air Safety Inves-
tigators, Dallas, TX, 1992.
Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 25