Accident Reconstruction
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Purpose / Aim
The purpose of the accident reconstruction is to allow investigators a better overall understanding of the accident event. An accident reconstruction involves investigation and analysis of an accident and the eventual drawing of conclusions that may provide investigators insight into contribution accident causation factors and resulting collision analysis.
The resulting data and analysis from an accident reconstruction can be used to improve the overall safety for vehicles, road infrastructure and road users.
Methods
All Teams
Accident reconstructions are instrumental in conducting an accident analysis. Furthermore, they also require specialist knowledge which all teams may not have access to. Therefore, to ensure that all accidents can be reconstructed (possibly by another team or an analyst using the data) it is important that all teams collect the required data to perform reconstructions accurately. Much of the data collected is merely useful when performing reconstructions, whereas other data is crucial and must be collected whenever possible, with high quality. Such vital information includes:
- A scaled sketch containing all relevant data
- Pictures of road layout, road user paths, traces, collision objects and vehicle deformations
- Information from the road users of their account of the accident
- A sequence of events according to the investigators
- Deformation measurements of the vehicles
Developing and Experienced Teams
As the investigation teams gain more experience, they will be able to find and interpret information useful for the reconstruction more efficiently. Examples of data that the teams should try to collect in addition to what is mentioned for all teams are:
- Roll over direction and number of rolls based on impact marks
- Braking or no braking based on deformations
- Basic speed assessment by the use of simple equations.
- Delta V based on energy based reconstructions
Experienced teams
The experienced teams should not only be able to collect and interpret all the information but use it for full reconstructions using specialized software. In cases were not all data could be collected the investigator will have to determine if the missing data can be replaced with estimates or if it is not possible to do a reconstruction. Based on the result of the reconstruction other collected should be reviewed and discussed if not in line with the reconstruction.
Deformation measurements for reconstructions
Interpreting data for reconstruction
Basic hand calculations
Energy based reconstructions
Full reconstructions using software
Theory of reconstructions
An accident reconstruction is based on three laws of physics, which have to be used by the investigator in order to define parameters, such as initial speeds and post crash speeds. These laws can be used separately (if only one variable is unknown) or combined (if more variables are unknown).
Newton's First Law of Motion
Every object in a state of uniform motion tends to remain in that state of motion
unless an external force is applied to it.
Newton's Second Law of Motion
The relationship between an object's mass 'm', its acceleration 'a', and the applied force
'F' is 'F = m x a'. Acceleration and force are vectors in this law and the direction of the force vector is the same
as the direction of the acceleration vector. 'F' is the acting force, 'm' the mass of the
body and 'a' is the acceleration of the body due to the acting force.
Newton's Third Law of Motion
Action and reaction are equal and opposite, i.e. when two bodies interact, the force
exerted by the first body to the second body is equal and opposite to the force exerted
by the second body on the first.
Newton defined the collision into two phases: the compression and the restitution
phase. In case of a full impact, at the end of the compression phase, the velocities of
both vehicles at the impulse point are identical. Due to elasticity of the vehicle
structures, the two vehicles will separate again.
Conservation of Energy
The conservation of energy states that the amount of energy in a closed system is constant,
regardless of the changes in form of that energy. Energy can neither be created nor
destroyed. Therefore the kinetic energy before the impact equals the kinetic energy
after the impact, in addition to the energy loss.
where:
m = the total mass of the bodies
v = the body velocities
i and j = the bodies involved in the crash
Energy can be lost during the impact due to:
- Deformation of vehicles
- Rotation of vehicle
- Friction between tires and pavement
- Sound due to impact
The energy loss due to deformation is more significant than the other values, as its magnitude is
much greater than the other losses. The other losses are difficult to define,
because of unknown parameters (e.g. duration of impact,
moments of inertia of vehicle, centre of gravity of vehicle). Since they are typically
one order of magnitude smaller, they are most often neglected. A parameter, which is
commonly used to define the deformation energy loss, is the Energy Equivalent
Speed (EES). The reconstruction parameter EES will be described later on. There are
crash test databases (the NCAP database for example) from which the EES can be
obtained.
Principle of Linear Momentum
Momentum is the product of inertia and velocity. During any collision, momentum is conserved as a consequence of Newton's 3rd Law - the Law of Action-Reaction. Momentum is the tendency of an object in motion to stay in motion. Thus, the total momentum before a collision is always equal to the total momentum after a collision.
A useful way of increasing the applicability of the above mentioned equation is by using the concept of elasticity. Elasticity is a measure of the ratio between the separation and final velocity. It can vary between 0 (fully elastic impact) and 1 (plastic impact, no separation).
Principle of Conservation of Angular Momentum
Angular momentum is the tendency of a rotating object to keep rotating at the same speed about the same axis of rotation.
Equipment
Please refer to the Equipment List for the Accident Reconstruction equipment list.
Arrangements
Please refer to the Arrangement section of the Methodology Outline Accident Reconstruction section.