Lesson 7
Forensic Engineering –
Vehicular Accident
Reconstruction
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Activity 7.1 Introduction
Short video on car crashes:
http://www.metacafe.com/watch/2382492/
car_crash_compilation
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Activity 7.2
Accident Scene Investigation
5,6
2
1
3,4
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Source: Tai Kung Pao (20-3-2010)
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Activity 7.2
Things you would do:
WHY ?
1. Examine the tyres of the vehicles
Flat or damaged tyres may cause the car to lose
control
2. Examine the road’s condition
Oil spill, strange obstacles and sharp objects may
cause the accident
3. Record the features of the skid marks
May show whether the tyre was flat or
overloading. (see Activity7.5)
4. Measure the length of the skid marks
May help estimate the speed of the vehicle before
impact. (see Activity 7.6)
5. To know whether the drivers are drunk or
medicated
Alcohol or drug will affect a driver’s reaction time
during an accident. (see Activity 7.4)
6. Obtain the oral confession of the eyewitnesses
They can provide information which are helpful in
reconstructing the accident.(see Activity 7.7)
7. Examine the hazard lights of the Rolls Royce
To check out whether the victim had turned on the
hazard lights before the collision. (see Activity 7.3)
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Activity 7.3
Reaction time of the driver
Reaction time is the lapse of time between stimulation
and the beginning of response.
Conditions
Reaction time
Normal
~ 1 sec
Having telephone
conservation /
Listening music
~1.5 – 2 sec
Drunk or under medication
~ 2 – 5 sec
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Activity 7.3
Reaction time of the driver
Thinking distance(思考距離) :
the distance travelled by the car in your reaction time
Braking distance(剎車距離) :
the distance skidded by the car after the brake is applied
Total stopping distance = Thinking distance + Braking distance
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Activity 7.3
http://www.shep.net/resources/curricular/physics/ja
va/javaReaction/index.html
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Activity 7.4
Features of skid marks
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Activity 7.4
Features of skid marks
Explanation :
under-inflated tyrecontact of tyre edges with road > (firmer than) contact of centre part of tyre with road
 The depreciation of the edges of tyre are much faster;
 When sliding on road, it gives skid marks with missing centre
part.
over-inflated tyrecontact of centre part of tyre with road > (firmer than) contact of tyre edges with road
 causing
skid marks with missing tyre edges
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Activity 7.5 START and STOP
The acceleration of a vehicle
Acceleration = the rate of change of the speed
vu
a
t
v  u  2a s
2
2
a = acceleration; v = final velocity; u = initial velocity; t = time interval; s = distance travelled
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Activity 7.5 START and STOP
The braking of a vehicle
Skid marks begin when the brake is applied
and end when the car stops.
It is the friction between the tyres and the
road surface that causes the car to
decelerate and stop.
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Experiment
To prove
F = constant x W
F
PULL
Figure 2
Figure 1
W
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Experiment
F
W
Drag factor f = F / W
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The braking of a vehicle
speed  254D f
Where speed : the speed just before sliding (measure in km/h)
f : drag factor
D : length of the skid mark (measure in m)
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After thorough investigation you get the
following information :
mass of the car and Carol : 1100 kg
 mass of Herny : 60 kg
speed limit at the location : 50 km/h
drag factor f = 0.7
point of impact : 28 m from the nearby
traffic lamps
17 m long skid marks from the point of
impact
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Herny lay on the ground;
clothes fibers and some blood spatters
were found near Henry’s body;
a few street lamps were out of order
during the accident;
the manufacturer of Carol’s car claimed
that the maximum acceleration of the car
is 2.4 m/s2
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Q1.
From the above information, can you
estimate the minimum speed of Carol’s
car during the crash?
(Since the mass of car is much heavier
than that of Henry, we can assume the
speed of the car didn’t change after the
collision.)
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Q1. Answer:
= 55.0 km/h
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In order to reveal the truth and provide
evidence in the court, your team had
interviewed Carol, Henry and Sam
respectively and got the following
confessions:
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Carol (the driver, a nurse, 40 , mother of 3
kids)
When I was hurrying to the hospital for work
that night, I stopped my car before the red traffic
light. The road was dark. After the traffic light
turned green and making sure that the road
ahead was clear, I drove across the road
junction. Suddenly, I saw a person wearing dark
clothes appearing in front of me. I swear that I
couldn’t see him until that moment. I pressed my
horn sharply but he didn’t seem to have any
response. Everything was too late and my car
knocked him down. I applied the brake
immediately to stop my car. I was so scared that
I stayed in my car until the police came.
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Henry (the injured person, a salesman, 26,
single)
After drinking with my friends, I left the
bar around midnight. I intended to catch
the last bus home. The pedestrian lamp
was green and flashing when I was
running across the road. Suddenly, without
sounding its horn a car at high speed
came to me from my right hand side and
knocked me unconscious.
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Sam (witness, a financial planner, 33,
married)
That was a quiet night. When I was walking
towards the bus-stop after meeting with a client, I
heard very loud sound of horn from behind
suddenly. I turned to my left and saw a car knocking
down a person. I remember that the pedestrian
lamp near the crash scene was still red at that
moment. I ran quickly across the road to see what I
could help. I saw a man wearing black clothes lying
on the road with some bloodstains around him. A
car stopped about 10 m away from the injured
person. The driver who was a lady got off her car.
She was very scared. I called 999 with my mobile
phone for her.
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Case study – midnight crash
Q2.
Comparing the confessions of Carol,
Henry and Sam list out some points that
you think are contradictory.
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Case study – midnight crash
Answers:
1.
Carol said that she drove across the road junction when the
pedestrian lamp was green, which is consistent with Sam’s
confession (“I remember that the pedestrian lamp near the crash
scene was still red at that moment.”). However, Henry said the
pedestrian lamp was green and flashing when he was running
across the road.
2.
Carol said that she had pressed the horn and the sound of horn
was also heard by Sam. However, Henry claimed that the car
came to him at high speed without sounding its horn.
3.
Carol said that she stayed in the car until the police came while
Sam said she got off the car right after the accident.
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Case study – midnight crash
Q3.
Comment on the confessions and discuss whether
they are trustworthy “()” or doubtful “(?)”.
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Case study – midnight crash
Q3. Answers:
Carol :
() The road was dark. (some street lamps were out of
order)
() I had pressed the horn. (supported by Sam’s
confession)
() I had braked the car after knocking down Henry.
(confirmed by the evidence from the skid marks left)
(?) I stayed in the car until the police came (contradicted
with Sam’s confession)
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Case study – midnight crash
Henry :
() I was running when I was knocked down by the car. (the inertia of
his body would bring him a bit north after the impact
resultant movement of Henry
running direction of Henry
movement given by car
(?) The pedestrian lamp was green / flashing (contradicted Carol’s
confession “After the traffic light turned green ( the pedestrian
lamp facing Henry should be red) and making sure that the road was
clear ……)
(?) The car did not sound its horn (contradicted Sam’s confession “I
heard very loud sound of horn from behind suddenly”)
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Case study – midnight crash
Sam :
() I heard the horn before the collision.
() The car stopped about 10 m from the body (quite
consistent with the site measurement)
(?) The pedestrian lamp was red during the collision (there
may be time lapsed (about 10 seconds), it is difficult to
prove that the pedestrian lamp was really red during the
moment of collision)
(?) The driver got off the car (contradicted with Carol’s
confession “I was so scared that I stayed in my car until
the police came”)
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Case study – midnight crash
Crucial factor
maximum speed she could attain is
v = 11.6 m/s = 41.8 km/h
Solution:
V2-u2=2as
V2-0=2as
V2=2as=2(2.4)(28)
V=11.6 m/s = 41.8km/h
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Carol drove across the road junction
when the traffic light facing her was still
red and hit Henry who was then running
across the road (the pedestrian lamp
facing him was green) ?
OR
Carol drove across the road junction
when the traffic light facing her was
green but hit Henry who was then
running across the road (the pedestrian
lamp facing him was red)?
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 However, the minimum speed of Carol’s car at
the point of impact was 55 kmh-1
 The conclusion is that Carol’s car did NOT start
from rest at the traffic lamp. In fact, she was
probably driving across the road junction when
the red light was on and her car knocked down
Henry.
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Activity 7.7 Accident Animation
 Nowadays, modern technology could develop
computer generated animations that accurately
represent the forensic testimonies and witness
statements to help the understanding of car
accidents or events. The animations pull
together all the relevant evidences and present it
in a logical and clear manner. They are able to
give a visual demonstration of events
surrounding the car accident. In some of the
cases, the animations are used in attending
meetings or providing courtroom presentation.
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Here are some examples of Accident
Animation (Multiple Animated Objects):
The animation is based on the footage of a
real accident. The animation includes the
Jogger View, Second Pedestrian View and
Car Across Intersection View. Luckily, the
person run over in the animation did
survive in the accident!
Reference for the animation :
http://www.mapscenes.com/software/captu
re/samples/index.htm
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