Computers
have affected the way we do business, and the area of automobile accident
reconstruction is no exception. The United States Department of Transportation
has developed several computer programs for automobile
Figure 1
accident
reconstruction. One of these computer programs is called Calspan Reconstruction
of Accident Speeds on the Highway or CRASH for short. This computer program
calculates the impact speeds and changes of vehicle velocity during an
accident.
Figure 1
shows typical information that is fed to the computer. The scene information
describes to the computer the locations of the vehicle rest position, the
expected impact position and direction of rotation. Next information must be
supplied to the computer concerning the characteristics of the vehicle (weight,
stiffness, model) as well as measurements of vehicle crush. The crush depth
along the damaged portion of the automobile is indicated by the variables C1
through C6. So in Figure 1, L indicates that the damage width is 65 inches
while the crush depths vary from 33 inches to 44 inches. The photograph in Figure
1 shows typical measurement of crush depth. Finally, the last piece of information
supplied to the computer describes the frictional properties of the road at the
time of the accident.
Figure 2
is the computer printout of the accident reconstruction. Under the printout for
vehicle number 1, the impact speed was 44 mph forward and 14.3 mph laterally or
to the right. This indicates that vehicle number 1 was attempting to turn right
to avoid the collision. The printout for vehicle number 2 shows a forward speed
of -2.2 mph which means the vehicle is moving backwards at 2.2 mph and a
lateral speed of 46 mph to the right, which means that vehicle number 2 was in
a sideways skid at 46 mph at impact. The speed change calculations indicate the
severity of the crash and are often used in determination of the chances of
occupant survival.
The
CRASH computer program is available from the National Technical
Figure 2
Information
Service and is also available from private software vendors.
The CRASH computer program is based on Newton's Laws of Motion and principles of
conservation of energy. It contains diagnostics that warn the user of improper
or unreasonable input data. The CRASH computer program is a useful tool for
analyzing the complexities of automobile accidents.
An Ingredient in Accident Reconstruction
Motor vehicle accident reconstruction is performed to
determine how an automobile accident occurred. It may be the primary means of
explaining what happened or a complement to eye witness testimony. An important
ingredient in accident reconstruction is the analysis of damage patterns on the
vehicles involved. Vehicular damage can yield information as to direction of
impacting forces, orientation of vehicles at impact, the nature of the other
vehicles or objects in the case of a hit and run, or as an indicator of injury
severity.
Accident reconstructionists utilize damage pattern information
to help determine the velocity change, an indicator of the severity of the
accident. The velocity change is defined as the velocity of a vehicle before
the impact minus the velocity of the vehicle after impact. This is often called
DV
(delta V) meaning a finite change as derived from differential calculus. If a
vehicle is traveling 20 miles per hour and strikes a solid wall such that the
velocity after impact is zero, then the velocity change is 20 miles per hour.
If a tractor semi-trailer unit is traveling on an interstate highway at 60
miles per hour and strikes a pedestrian, the tractor trailer may slow to 59 mph
while the pedestrian will accelerate from 0 to 59 MPH. The semi has a velocity
change of 1 mile per hour, hardly damaging to a semi, while the pedestrian sustains
a velocity change of 59 miles per hour, usually fatal to a pedestrian. (See Computerized Accident Reconstruction,
Page I-3 for numerical calculations of DV).
The following is a review of several examples of vehicle
deformations from accidents, along with a qualitative assessment of velocity
change.
Figure 1
Figure 1 is a view of the typical "fender
bender" frontal impact with a non-compliant object such as a pier used to
support a light pole. The force causing the deformation is from the 1 o'clock position (12 o'clock is directly down the long
axis of the vehicle from front to back). Because of relatively little deformation,
one can conclude that this is a low velocity change accident.
Figure 2 is a view of the other extreme, a high speed
impact with a relatively non-compliant embankment. In this single vehicle
accident, severe deformation is noted on the left front of the vehicle, indicative
of high velocity change. The principle force direction is from the 10 o'clock position toward the
vehicle.
Figure 2
Figure 3a
Figure 3b
Figures 3a and 3b show views of a typical sideswipe
accident. The left side of the vehicle in Figure 3a has contacted the left side
of the vehicle in Figure 3b near the wheel well. Despite the long damage
pattern on the left side of the vehicle in Figure 3b, this is a low velocity
change accident since relatively little vehicle velocity is exchanged for
crush. The principle force direction is from the 12 o'clock position on vehicle number 1 and from the 11 o'clock position on vehicle number
2.
Figures 4a and 4b show the outcome of an off-set frontal
impact accident.
Figure 4a
Figure 4b
Damage patterns show severe crush of engine compartments
nearly halfway to the bulkhead or fire wall. This is a relatively high velocity
change accident. The force direction is from the 12 o'clock position in each vehicle.
Figure 5 shows the typical perpendicular impact, often
called the "T-Bone" impact. Vehicle number 1 has impacted number 2
with high velocity change in
Figure 5a
Figure 5b
each case. The force direction is from the 12 o'clock position on vehicle number
1 and the 9 o'clock position
on vehicle number 2.
Figures 6a & 6b show the classical rear end impact
where vehicle number 1 has rear ended vehicle number 2. This one is a
relatively low velocity change impact with the force directions from the 12 o'clock position of vehicle number
1 and the 6 o'clock
position from vehicle number 2. Because of its relatively high stiffness,
vehicle number 1 has sustained little damage.
Figure 6a
Figure 6b
Figure 7
Figure 7 shows the typical
under-ride accident. During this type of accident, the upper softer structure
of the vehicle absorbs the impact as opposed to the lower, stiffer structure.
There is often significant penetration into the occupant compartment and
serious injury or death. Despite the severe damage to the vehicle, under-ride
accidents are often low velocity change when only the upper vehicular structure
is involved. Here the force direction is from the 11 o'clock position.
Figure 8
Figure 8 shows deformations resulting from a roll-over.
Roof crush is significant on the right, indicating an impact point during
roll-over. Severe scraping on the roof is another indication of a roll-over.
Velocity change is not often analyzed on roll-overs since the process of
rolling does not decelerate the vehicle in the typical 200 millisecond crash duration
found in other types of impacts. Occupant injuries usually result from repeated
impacts with the vehicle interior over a period of seconds and from possible
expulsion from the vehicle.
Figure 9 is a view of a dump trailer showing severe
deformations to the frame and severe deformations to the front of the dump box.
The vehicle was traveling at about 55 MPH on an interstate highway when the
dump box began to rise as the vehicle approached an overpass. The front of the
dump box struck the overpass while in the raised position, tearing it loose
from the fifth wheel. Because of the massive overpass,
Figure 9
the dump box stopped immediately (DV of approximately 55
MPH). Even though vehicles were trailing at a “safe distance” behind the dump
box, impacts did occur with the dump box. Most safe following distances assume
that the vehicle in front has a finite stopping distance similar to the
following vehicle. When the stopping distance is very small, as in this case,
an impact can occur even when following at a "safe distance."
Figure 10
Some accidents involve vehicles with substantially
different weights. Figure 10 shows a school bus after impact with a bicycle
with the bicycle engaged under the bus frame. There is virtually no damage to
the bus, while the bicycle is severely damaged. Velocity change calculations are
difficult in this case, since impact deformations to the bicycle are eclipsed
by more severe deformations from crush from the bus undercarriage.
Figure 11 is the classic utility pole impact deformation.
Figure 11
Damage patterns on motor vehicles are often indicative of
the velocity change sustained during an accident. Accident reconstructionists
use this information to calculate impact speed and orientation of vehicles.
Claims personnel can significantly contribute to an accident investigation by
using good photographic techniques. Photographic frontal and side views of the
vehicles using a 35 mm camera are very helpful to the accident reconstructionist
after the vehicles have been salvaged. It is very discouraging to analyze an
accident several months after a loss using a few "fuzzy" photographs
from an instant camera.
Damage patterns on vehicles tell a story as to accident
dynamics. An inspection of vehicular damage by claims personnel may support or
conflict with versions of how the accident occurred. Damage patterns on
vehicles also help explain deployment or non-deployment of an air bag.
Damage pattern analysis is useful in hit and run cases
where the offending vehicle is unknown. Interpretation of impact damage is
often a deciding factor between denying or extending coverage.
Truck
accidents often result in considerable property damage because of their size
and the value of cargo transported, not withstanding personal injury. When
working a file involving a truck accident, an assessment of the probable cause
of the accident will be essential to the final report. The following is a review
of typical causes of truck accidents and some comments that may help in
formulating a report.
Automobile driver error: Truck accidents have occurred when
automobiles collide with trucks as a result of failure to yield, driving under the
influence, and inattentive driving. The police report, statements, photographs
of the accident scene, photographs of the vehicles and an accident reconstruction
may be helpful.
Truck driver error: Truck accidents have occurred as a
result of drivers falling asleep, driving under the influence of alcohol (and
sleep inhibiting drugs), insufficient training, improper vehicle handling
procedures and inattentiveness.
Figure 1
The
vehicle shown in Figure 1 was damaged as a result of improper vehicle handling.
Driver's log, medical reports, police reports, statements, photographs and an
accident reconstruction may help piece together what happened. In one instance
a driver upset a large dump trailer causing a large loss. It was later determined
that the driver attempted to dump the load with the tractor in a jackknifed
position contrary to accepted practices and manufacturer’s recommendations.
Load Shift: Trucks carry large loads that are
required to be properly secured to the vehicle. In some instances due to chain
failure or dunnage compression, the securing straps or chains may loosen,
allowing the load to shift. If the truck stops suddenly or goes around a corner,
an upset may result with considerable
Figure 2
property
loss and personal injury. Figure 2 is a view of a load shift related accident. Photograph
the accident scene and cargo. Take measurements of the rest position of the
truck and cargo for accident reconstruction. Save pieces of packing material
that may show improper packaging. The accident may have been caused by an
improperly packaged or secured load.
Brake Failure: Brake failure can occur as a
result of poor maintenance, corrosion or mechanical failure. A detailed inspection
of the brake system is necessary to determine the existence of a fault in the
brake system. Figure 3 is a view of
Figure 3
an
inspection of a truck brake system that was found to be improperly adjusted and
a cause of an accident. Inspect and photograph brake components as soon as
possible. Operate the brake system, if possible, by pressurizing the receiver
tank and look for push rod actuation from brake chambers. Save failed parts
removed from the system. In one instance, it was discovered by inspection that
three of the four brake assemblies on the tractor were improperly adjusted.
Only one brake was doing all the work, which was adequate for typical braking.
However, during an emergency stop, insufficient braking was available, resulting
in personal injury and significant property loss.
Tire failure: Blowout of a tire, especially a
steer tire on the tractor, may result in loss of vehicle control and an accident
as shown in Figure 4. Save the tires for a tire expert to determine if the tire
failure is a result or cause of the accident. Photograph the accident scene and
the vehicle since this helps the tire failure analyst in the determination of
the probable failure cause.
Mechanical failure: Mechanical failures due to corrosion,
such as the failure
Figure 4
Figure 5
of the
truck frame in Figure 5, improper maintenance, design defects and production
defects do occur. Good photographic documentation is very helpful in the
failure analysis. Since truck components are very large, it may be impractical
to remove them immediately to storage. Obtaining statements on how the accident
occurred, reviewing manufacturer's recall notices and inspecting a similar
vehicle can yield important information for the report.
INTRODUCTION
Response time is the total time it takes for a vehicle
driver to perceive, evaluate, decide and react to a situation on the roadway.
Since vehicle driver response time can be as long as 2-4 seconds, highway
designers, accident reconstructionists and the courts take this into account. A
highway designer allows several seconds of unobstructed view of a traffic
control sign, giving time for the motorist to respond to the sign information.
An accident reconstructionist utilizes response time to explain why an accident
occurred, i.e.: a bicyclist suddenly pulled in front of an automobile, giving
insufficient time for the motorist to respond and avoid the accident. The
courts are interested in apportioning liability to various parties in a legal
action based on response time. At issue, of course, is the question, "Was
there sufficient time to avoid an accident?" Claims professionals deal
with this issue on a daily basis. Claimants often argue that the insured caused
an accident by giving little time to respond. Other losses involving personal
injury and property may be a consequence of a vehicle traveling at a high rate
of speed leaving insufficient time to respond to road situations.
Like many other human characteristics, response time is a
highly variable quantity. For the same road hazard, the response time of
vehicle drivers varies markedly as a function of age, time of day, weather
conditions, chemical ingestion, and fatigue. For a given driver, response times
vary depending on type of hazard.
WHAT IS RESPONSE TIME
The studies of psychology and ergonomics have delved into
the intricacies of how a person responds to stimuli. Current thought considers
response time to be a summation of times required to activate biological
functions. For instance, if one views a hazard on a roadway, there is a time
required for conversion of the optical image into a nerve impulse, time to transmit
a signal along a nerve to the cerebral cortex, time for processing of the
signal by the brain, time for transmission of a signal along a nerve to musculature
and time delay of muscle response. The sum total of these times is often called
the response time, a term often confused with reaction time.
Figure 1
Figure 1 shows a driver sitting at a test apparatus that
evaluates reaction time. As soon as the light turns red on the console, the
driver releases the accelerator and applies the brake. The reaction time is
measured. This form of testing is often called simple reaction time, as it is a
result of a single stimulus, the red light. Reaction times are typically on the
order of 3/4 of a second. However, response times are more complex and can be
as high as 3-4 seconds. So what makes up the difference? The answer is the
perception/decision time. The following equation shows the components of
response time:
Response time = “Perception/Decision” Time
+ Reaction Time
The “perception/decision” time is the time it takes to
view a hazard and figure out what to do about it. The reaction time is the time
it takes to perform a particular function once a decision has been made. The
response time for removing one's hand from a hot skillet is relatively quick
and is on the order of about a half second. In this example, a natural response
to excessive heat bypasses the visual sensors, allowing for a quicker response
time. Driving an automobile requires a high degree of visual processing, which
tends to extend response times.
McGee et. al. (1) reported that perception time is the sum
of eye movement time, fixation on the hazard time delay, recognition time delay
and muscle response delay time. They found that for the 85th percentile
of drivers, eye movement delay was 0.09 seconds, fixation delay time was 0.20
seconds, recognition delay time was 0.50 seconds, decision time 0.85 seconds,
muscle response delay was 0.31 seconds and brake reaction time was 1.24
seconds. The sum total of these times, the response time, was 3.19 seconds. The
85th percentile is often chosen as the upper bound for design
analyses.
Many such studies assume that each of the component times
are additive to form the total response time. Some investigators report that in
many situations, parallel processing occurs, indicating that some of the
component activities may be occurring simultaneously, reducing the calculated
response time. Also, the type of hazard being recognized plays a major role in
response time. For instance many laboratory tests involve a person sitting at a
vehicle simulator waiting for a light to signal when to press the brake pedal.
This type of testing usually results in short response times since the
participants know what to expect and are ready. Contrast this with a driver
cresting a hill and viewing a semi-tractor trailer rig broken down in the right
lane of traffic, in a no parking area without signals or markers deployed. At
the time of initial visual fixation, it may not be obvious that the vehicle is
not moving, which may extend the perception/decision time significantly.
USAGE OF RESPONSE TIME
Figure 2 is a view of an accident scene showing long tire marks
and an impact point at an intersection. Vehicle number 2 had driven across the
roadway and was struck by vehicle number 1. The tire
Figure 2
marks of vehicle number 1 prior to impact measured 234
feet in length. The distance from the crest of the hill was 500 feet. The speed
limit was 45 MPH. If vehicle number 1 had been traveling the speed limit (45
MPH, 66 ft/sec), it would have taken approximately 7 seconds to reach the
stalled truck, if no braking occurred. Being a clear day with ideal road conditions,
a response time of 3 seconds would appear attainable. Consequently as vehicle
number 1 crested the hill (assuming a 45 MPH speed), approximately 198 feet
would be traveled before active braking occurred, with 302 feet left in which
to stop. At a speed of 45 MPH, assuming a drag coefficient of 0.6 (a typical
value for dry asphalt), the vehicle could be brought to a stop in approximately
114 feet, which is well within the 302 feet available. Therefore, if the
vehicle had been traveling at the speed limit, the accident would have been
avoidable. The 234 feet of tire mark suggests that the vehicle was traveling at
a minimum of 64 MPH (94 ft/sec) at the beginning of the tire mark (again using
a drag coefficient of 0.6). Assuming a response time of 3 seconds and a speed
of 64 MPH, approximately 282 feet were traveled by vehicle number 1 before
brake application occurred. The remaining distance of 218 feet was not
sufficient to stop before contacting the stalled truck. Consequently, if the
driver of vehicle number 1 had been traveling the speed limit, the accident
probably would not have occurred. The stalled truck, crossing the intersection,
is an obvious hazard and it can easily be seen that the vehicle is not moving.
Figure 3 shows a slightly different scenario. A tractor
trailer rig has broken down on a two lane highway with no markers or emergency
lighting activated. The speed limit is 45 MPH. The distance from the crest of
the hill to vehicle number 1 is 300 feet. From the rear, it is not obvious that
the tractor trailer is stalled since no markers or warnings are deployed. Using
a response time of 3 seconds, vehicle number 1 would have traveled
approximately 198 feet (at 45 MPH) before brake application leaving 102 feet
remaining for stopping. The stopping distance at 45 MPH is approximately 114
feet, not enough distance to stop without impact with the truck. One may argue
that a 3 second response time is excessive and that perhaps a 1.5 second
response time is
Figure 3
more realistic. However, the one complicating factor in
this accident scenario is the non-obvious nature of the parked tractor trailer
rig. Without warnings deployed, there may be difficulty in the determination of
whether the vehicle is moving or not. This can significantly extend the perception
time, such that a 3 second or higher response time would be reasonable.
OTHER INFLUENCES AFFECTING RESPONSE TIME
A little more skill is required to operate a motorcycle
when compared to an auto-mobile, consequently the separate licensing requirements
in several states. In sudden hazardous situations, “slamming” on the brakes can
have a detrimental effect, often causing the motorcycle to fall to one side or
causing the operator to be catapulted over the handlebars. Consequently, application
of the rear brake only is a prudent means of avoiding an accident when driving
a motorcycle. The decision to use rear brakes only can increase response time.
Also with only the rear brake being activated, the stopping distance increases
since about 50% braking is being applied. What may be an appropriate response
time and stopping distance for an automobile may not be appropriate for a motorcycle.
It has been well documented that visual functions decrease
in proportion to decreasing illumination. Dirt on headlamps and crazing of
windshields over time also aversely affect visual acuity. Headlight alignment
greatly affects the perception of pedestrians as well as other vehicles.
A person's age affects response time with increases being
most notable in the later years. An example of an age effect is the need for
reading glasses or bifocal lenses. The growing inflexibility of the lenses in a
person's eye causes this condition (presbyopia). Visual acuity typically peaks
at about age 15 and declines to about 33% of the highest value at age 80
(Reference 2). The elderly have been found to be much more adversely affected
due to their loss of acuity and other visual functions. Typical studies show
reactions times of drivers near the age of 70 increase by approximately 20% over
those for age 20 (Reference 3).
Chemical usage has been shown to have a substantial effect
on response time. Reference 4 reports that at a blood alcohol content of 0.02%
by weight, the average increase in errors in simulated driving is approximately
6 in the particular study. At a blood alcohol content of 0.08%, the average
increase in errors in simulated driving is approximately 25 in this particular
test. Therefore, the simulated driving error rate at 0.08% blood alcohol
content was 4 times that at 0.02%, indicating that, despite the fact that a
driver may have a blood alcohol level below a legal limit, there is still an
effect on the response time.
Reference 5 reports that the average reaction time for
women was approximately 15% longer than for men.
CLOSING
What can be gleaned from the previous discussion is that
response time is a distributed quantity because of variability in people, as
well as in situations that require a response. The accident reconstruction
community often assumes a maximum 2.5 - 3.0 second response time. This may be
applicable for most accidents with obvious hazards. Other accidents involving
less defined or confusing hazards may result in longer response times. Other factors
extending response time are age, time of day, gender and chemical usage, suggesting
that response time is typically characteristic of a particular set of circumstances
encountered in an accident.
REFERENCES
1. McGee,
et. al., Highway Design and Operation Standards Affected by Driver
Characteristics, Volume II: Final Technical Report, Bellomo-McGee, Inc. Vienna, Virginia,
Report No. FHWA-RD-83-015, 1983.
2.
Verriest,
G., L'influence de l'age sur les fonctions visuelles de l'homme, Bulletin de
L'Academe Royale de Medecine Bellique, 1971, 11, 527-578.
3. American
Automobile Association, Traffic Engineering Safety Department, Age and
Complex Reaction Time, Report No 41, 1952.
4. Barzelay
and Lacy, Scientific Automobile Accident Reconstruction, Matthew Bender, New York, 1985.
5. American
Automobile Association, Traffic Engineering Safety Department, Reaction Time
as Related to Age, Report No. 69, 1966.
And Their Effect on Motor Vehicles
Some vehicular accidents are a result of road surface
conditions. Potholes, road grooving, bumps and rocks are some examples of road
hazards that have been know to cause accidents. Evidence of a road surface
induced accident may arise from driver statements, vehicular damage and
inspection of the scene. Driver
Figure 1
statements may contain claims of striking a road hazard or
blaming a vehicular mishap on road conditions. The validity of the drivers’
versions of accidents should not be immediately accepted as fact by the analyst
without reviewing additional data, if available. For example, if a driver
claims he lost control of his vehicle because of striking a pothole, yet the
road conditions were found to be extremely icy at the time, it is possible that
the icy conditions alone may have been the cause of the accident. Driver
statements are an important part of accident reconstruction especially when
physical evidence is consistent with the statement.
Examination of the scene yields physical evidence, such as impact marks, tire marks, vehicle
deformations and road irregularities. Photographs of the road irregularities
help document the scene as shown in Figure 1. Photographs of damage to vehicles
consistent with having impacted a road discontinuity, bolster
Figure 2
conclusions as to the influence of the road hazard. Figure
2 is a view of typical wheel rim impact damage. If a pothole is suspected of
having precipitated an accident, measurement of the dimensions may be helpful.
Measurement of its length along the direction of travel plus the average depth
can be used to assess the extent of the hazard. Figure 3 is a graph presented
in a report entitled "The
Figure 3
Influence of Roadway Surface Discontinuities on Safety,”
Transportation Research Board, National Research Council, 1984. In this graph
one can rate the severity of a pothole and its likely effect on an
accident. Four test vehicles of various
sizes were used in the referenced study to generate the graph. The relatively
safe region on the graph contains length/depth dimensions that did not appear
to cause handling problems or damage to the vehicle. The questionable safety
zone on the graph captures length/depth dimensions that could result in
vehicular damage. Finally the unsafe zone encompasses those length/depth
dimensions that can produce severe damage to the vehicle or loss of control.
For instance if a pothole measures 125 inches in length and 6 inches average
depth, then this pothole may have had an effect on vehicular control or damage
at speeds from 20 to 60 MPH. If the pothole measures 100 inches in length and 2
inches in depth, then it is unlikely that it had any effect on an accident. It
is interesting to note by the graph, that a given size pothole has a larger
unsafe zone for slower speed vehicles than for faster vehicles. Unlike many
other highway hazards, potholes may result in more vehicular damage at lower
speeds for a given pothole. This is partially a result of the response of the
vehicle suspension system to the particular hole. Some pothole related
accidents occur from drivers attempting to avoid a pothole without actually
striking it.
Reference
“The Influence of Roadway Surface Discontinuities on
Safety,” Transportation Research Board, National Research Council, Washington D. C., 1984.