Forensic Engineering

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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
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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 roads 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 drivers 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(????) the distance
travelled by the car in your reaction
time Braking distance(????) 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/j
  ava/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 tyre-
• contact of tyre edges with road gt (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 tyre-
• contact of centre part of tyre with road gt
  (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

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

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Experiment
F
W
Drag factor f F / W
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The braking of a vehicle

• 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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(No Transcript)
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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 Henrys body
• a few street lamps were out of order during the
  accident
• the manufacturer of Carols 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 Carols car during
the crash?(Since the mass of car is much heavier
than that of Henry, we can assume the speed of
the car didnt 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
  couldnt see him until that moment. I pressed my
  horn sharply but he didnt 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
• Carol said that she drove across the road
  junction when the pedestrian lamp was green,
  which is consistent with Sams 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.
• 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.
• 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 Sams
  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 Sams 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 Carols 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
  Sams 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
  Carols 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-u22as
• V2-02as
• V22as2(2.4)(28)
• V11.6 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 Carols car at the
  point of impact was 55 kmh-1
• The conclusion is that Carols 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/capture/samples/index.htm