Forensic Engineering – Vehicular Accident Reconstruction

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Forensic Engineering – Vehicular Accident Reconstruction
Lesson 7 Forensic Engineering – Vehicular Accident Reconstruction

Activity 7.1 Introduction
Short video on car crashes:

Activity 7.2 Accident Scene Investigation
5,6 2 1 3,4 7 Source: Tai Kung Pao ( )

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)

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

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

Activity 7.3

Activity 7.4 Features of skid marks

Activity 7.4 Features of skid marks
Explanation : under-inflated tyre- contact 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 tyre- contact of centre part of tyre with road > (firmer than) contact of tyre edges with road  causing skid marks with missing tyre edges

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

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.

Experiment To prove F = constant x W Figure 2 PULL Figure 1 F W

Experiment F W Drag factor f = F / W

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)

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

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

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.)

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:

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.

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.

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.

Case study – midnight crash
Q2. Comparing the confessions of Carol, Henry and Sam list out some points that you think are contradictory.

Case study – midnight crash
Answers: 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. 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.

Case study – midnight crash
Q3. Comment on the confessions and discuss whether they are trustworthy “()” or doubtful “(?)”.

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)

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”)

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”)

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

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)?

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.

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.

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 :

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