biomechanical analysis in accident reconstruction
PhD P.Eng. Uzi Raz
In many accidents, the question of who drove the vehicle arises. In these cases, the reconstruction involves a deep analysis of the mechanics of the vehicle, the damages to the vehicle and the road conditions. The result of this analysis is a set of force vectors which apply to the occupants of the vehicle. There will be a different set of force vectors for each occupant. The occupants of the vehicle have to be examined for their reaction to this set of force vectors. Their injuries have to be the results of these force vectors. Matching the injuries with these force vectors reveals the true location of each wounded person prior to the accident.
A case study is presented to demonstrate the above method.
It is important to identify the driver of a vehicle which was involved in an accident because of both the criminal (traffic laws) and the civil (insurance and damage claims) litigations. After some accidents in which a few or all occupants have been injured or died, the persons responsible for the investigation cannot answer the question: who drove the vehicle before and during the accident. In some of these cases, the people involved are not willing to disclose this information because of personal reasons such as: insurance fraud (the driver had no insurance for the vehicle he was driving) or criminal liability (accusing the dead occupant relieves the driver from his/her responsibility).
In cases like this, a different approach has to be utilized to find the identity of the person who drove the vehicle during the accident.
Data on injuries of car occupants in frontal or rear end collisions have been collected and presented.
This data constitutes a "catalog" of typical injuries of either the driver or the passengers. When investigating a frontal or rear end collision, it recommended to cross reference the injuries of the occupants of the vehicles involved in the accident with the outcomes of accidents presented in this "catalog".
However, when the accident is due to loss of control and is not a frontal or rear end collision, this data is not pertinent.
Since there is not enough data on such accidents, it is necessary to examine each accident in particular methodology. This way will enable us to distinguish between the driver and the passengers.
Each case will have to be examined according to the laws of physics (the kinetics and the dynamics of the accident) and only solutions that are physically possible will be considered. The examination should include vehicle dynamics and damage pattern, as well as injuries to all the occupants. In these cases, the forensic medicine specialist or other experts such as radiologist, orthopedic surgeon etc. should be consulted to help compare the injuries, both internal and external with the damages to the vehicle involved.
In order to solve this problem, the investigator should work methodically, obtain all the information from the scene of the accident, from the vehicle involved and from the medical charts as well as consulting the medical personnel.
In order not to miss any information, it is suggested for the investigator to go through the following steps:
Reconstruct all the dynamic and kinematic data of the vehicle prior and during the loss of control, up to the first impact point.
Reconstruct all the dynamic and kinematic data of the vehicle from the first impact point to the final rest position.
Find the inertia vectors that operated on the vehicle's occupants through the first two stages.
Compare the set of inertia vectors operating on the occupants with their motions in the vehicle during the accident.
Determine the body movement possibilities inside the vehicle for each seat.
Compare the damages to the interior of the vehicle with the injuries to the occupants of the same vehicle.
Consult the medical experts to ensure that the damages to the interior of the vehicle could cause these injuries.
Note: In all stages, when applicable, known appropriate computer simulation programs are very useful.
In order to demonstrate this procedure, a real case will be presented. The general description of this accident is: a car with three occupants lost control and skidded into a lamp post. One passenger, a young man, in the rear seat was captured there and was badly injured. One occupant, a young man, was thrown out of the vehicle and was injured. One occupant, a young girl, remained in the vehicle, badly injured and ended up paraplegic.
In court, both civil and criminal, the question of who drove the vehicle during the accident was raised. Both front seat occupants claimed that they were not driving during the accident.
Reconstruction of the dynamic and kinematic data.
When a vehicle enters a curve, it moves in a circular path according to the steering so that the centre of gravity of each wheel is moving in a different circle, see figs.1&2. Normally, when the vehicle is under control, the circles are concentric. The force causing the rotation (rather than moving straight) is the centripetal force created by the friction between the wheels and the road under the vehicle's weight. The reaction force, the centrifugal force, is the force of inertia. Both forces work in opposite directions, to and from the centre. However, when the centripetal force disappears, the vehicle will move in accordance with the inertia of the vehicle, in the tangential direction of the circles, in a straight line.
Most accidents happen when the body of the vehicle moves in a compound movement of both straight and curved lines. The speed of the vehicle is also changing either accelerating or decelerating. This is yaw movement in which the vehicle goes both straight and in rotation around its centre of gravity. This happens when the wheels of rear axle lock or when the friction factors between the wheels and the road are different for each wheel. When the force causing this particular movement disappears, the vehicle moves straight in a line created by the complex movement, which has to be calculated in each case.
Since the occupants of the vehicle are the subject of this investigation, the movements of each one have to be considered and calculated separately, within the constrictions of the vehicles movements.
The point of impact is the point where the external forces on the vehicle (road adhesion) disappear because of the crash. The occupants are subject to a force vector due to the inertia vector of each of them. Thus they continue their movement in a tangential line to their movements at the instant of the impact. The body of each occupant will move until it will hit a part of the vehicle which will stop its movement, either by restraining it (belts or airbags) or by barring it.
Analyzing the accident from the impact point to the final rest position.
The analysis of the accident is done utilizing all the data collected at the scene of the accident by the police or other investigators. Additional helpful data is collected by the insurance loss assessors and the workshop repair personnel. The damages presented on the vehicle and shown on the pictures may be caused by two different mechanisms. One type of mechanisms is the accident itself. The second type of damages is caused by the rescue crew who are more concerned with life saving than with collecting data for the investigation of the accident, see fig.3.
The force vectors that caused the damages of the vehicle can be derived from the damages of the accident, see fig. 4.
If the damages found are in different directions than the original accident, the possibility that they were caused in different mechanisms and times must be considered. To demonstrate such cases, see figs. 5&6.
All interior damages have to be examined if they were created by the occupants, for example see figs. 7&8.
Sometimes, the interior damage can be misunderstood for a damage caused by an occupant of the vehicle rather than by an external body, see figs. 9&10.
In the case presented here, the vehicle was found with two main body damages located on the left side of the vehicle and a secondary damage located on the left centre pillar, between the left doors. The centre of the main damage was behind the central left pillar of the vehicle. This damage has a circular shape. This damage is deep and extends to the roof of the vehicle, see fig.11. This damage was caused when the moving vehicle collided into the lamp post, at the end of the yaw movement. This damage shows the movement of the vehicle from the first impact point to the final rest position.
The second main damage was to the left front door (driver's door). The shape of this damage is a sharp bend in the centre of the door. The damage direction is from the inside of the car toward the outside. The damage can be distinguished on both sides (the interior and the exterior) of the door, see figs. 6&7. It can be deduced from this shape that the force causing it was from the inside towards the outside of the vehicle. Since the vector that caused this damage was in opposite direction to the vehicle's accident vector and the direction of the damage does not coincide with the accident damage, it can be safely assumed that this damage was caused by one of the occupants of the vehicle.
The secondary damage is a clockwise twist of the central pillar in an opposite direction to the main body damage. The centre of this twist is the locking mechanism of the door to the pillar, see fig. 12.
The driver's door was found open after the accident while the other doors had to be opened by the rescue team. No safety belt was cut during the rescue and there were no visible stretch marks on the safety belts, indicating that they were not used by any occupant prior to the accident.
The interior of the vehicle was moved during the rescue operations therefore, by the time the investigators collected the data from the vehicle's interior, it was virtually impossible to distinguish damages to the gearshift column or the front panel.
The inertia vectors that operated on the vehicle's occupants through the accident
To understand the movement of the people inside a vehicle during a crash or loss of control, the investigator has to be familiar with the laws of physics in the following areas: inertia, friction, momentum and impulse. The motion of the vehicle occupants can be calculated using kinetics equations. However, the movement of the people in the vehicle during a crash lasts a very short time, in the region of 25-150 milliseconds.
At the end of the crash, the car occupants are usually either restrained by the safety restraint system of the vehicle or they collide with the internal parts of the vehicle and sometimes are even thrown out of the vehicle.
The investigator has to review the data of the injuries of the occupants, as supplied by the medical staff and compare them to the damages of the vehicle.
To be able to do so, the investigator has to find each of the movement vectors of the car passengers and driver of the vehicle. To achieve this, he has to plot the movement of the occupants inside the moving vehicle and continue the lines of the movement at the time of the crash at a tangential direction to their last movement before the crash. In the case presented here, the movements of the two front seat occupants were plotted, as shown in fig. 8. It can be seen that although the two occupants were moving at different circles, the circles were similar enough to determine that the vectors of movement were in an angle both to the left and to the front of the vehicle.
Comparing the damages of the car to the injuries of the occupants
The occupants were injured because their body organs hit parts of the vehicle.
The passenger in the back left seat was subject to the same vectors as the driver. However, the lamppost damaged the vehicle so that the rear left door was pushed in an angle towards the inside and the rear of the vehicle, see fig. 13. His injuries were very severe since he moved towards the door but the door moved in the opposite direction, towards him. Because of that he suffered a severe trauma to his left side.
The driver's door was hit from the inside with a vector towards the outside. The twist to the central pillar between the left doors and the fact that the driver's door opened during the accident reflects on the type of injuries possible to the driver. In this situation, the driver must have been thrown towards the left front door therefore, his left side will show signs of an impact. When the driver impacted the left door, the door buckled and opened and he was thrown outside of the vehicle, hitting the tarmac and getting injured again. The driver's injuries were less severe than those of the passenger sitting behind him because the door yielded and he had a longer stopping distance. A longer stopping distance means less deceleration hence, weaker forces.
The front seat passenger was also thrown left. However, her legs, which were very close to the front panel had a very limited space to move, were restrained by the gearshift column as well as the front panel, see figs. 14&15.
A human body is not a solid object and is built of many joints. In this accident, the torso moved even though the legs were restrained. The torso moved towards the left but the legs could not move therefore, the torso twisted while the lower spine acted as the axis of the twist. At the resting point, the head hit the steering wheel and part of the torso was in the left side of the car. In this movement, when the torso moved in an angle to the front and left, while the legs moved only forwards, the lower vertebras of her spine were twisted. Due to this twist, the spine was separated between D11 and D12 and the spinal cord was cut, as shown in figs. 16&17.
As a result of these injuries, the passenger became paraplegic.
A methodical examination of an accident scene and an analysis of the mechanics of the vehicle, the damages to the vehicle and the road conditions results in a set of force vectors which apply to the occupants of the vehicle. There will be a different set of force vectors for each occupant. Comparing these vectors with the injuries sustained in the accident will enable the investigator to determine the location of each occupant of the vehicle during the accident. In this case, it was determined, with the aid of Dr .Md. R. Starinsky (Asaf Haroffe medical centre) who considered the injuries of both occupants that:
The driver was thrown out of the vehicle.
The passenger remained in the vehicle, partially on the driver's side.
This case was tried in court in Israel and the verdict was in accordance with the findings presented here.
It is important to point out that, had this person been tied with a safety belt, her torso would have been restrained so it could not have twisted and the damage to the spine would have been avoided.
Injury risk of occupants and pedestrians injury mechanism and protection criteria
EVU proceedings Nov. 2008
Cheng,H.; Rizer. A.L.; Obergefell, L.A.
Articulated total body model version V
Lynn B. Fricke and Kenneth S. Baker
Understanding occupant behavior in vehicle collisions.
Northwestern university traffic institute 1990
Ratnibd N, Brach and R. Matthew Brach
Vehicle accident analysis and reconstruction mathods.
SAE international 2005
Lawrence E. Goodman and William H. Warner
University of Minnesota 1985
PhD P.Eng. Uzi Raz
The Israeli college for security and investigation