A.D. Tupper & Associates Limited

When two vehicles come together, the crush on one car is produced by the forces from the other car, and vice versa. By examining the nature and extent of the damage on a vehicle, it is possible to determine the location and direction of the principle direction of force that acted against that vehicle. This force, which is typically shown with an arrow vector, is known by reconstructionists as the PDOF, or principle direction of force.

It is an elementary principle of physics, embodied in Newton’s first law of motion, that for every action there is an equal and opposite reaction. This means that when two vehicles come together, their PDOFs must be aligned and opposite in direction.

At the point of maximum engagement, the maximum crush on one vehicle has to match the maximum crush on the other vehicle. The effect of this, therefore, is if scaled plan drawings of the damaged cars are positioned so that their damaged areas overlap and are coincident, the relative position of the vehicles at the point of maximum engagement can be established.

Having regard to the direction and nature of the damage inducing forces, and having regard to the first law of motion, it is then possible to work back and determine the relative positions of the vehicles at the point of initial impact.

These relative positions are dependent upon the damage and the nature of the collision forces, and are fully independent of the geography of the collision scene.

Often myriad marks are left before, during and after a collision event. By determining what element of a vehicle made a particular mark, it is thus possible to anchor that piece of the vehicle to the ground, and to establish where that car was at a particular point.

In the limit, for instance, if there are full skid marks from both vehicles, then the cars can be positioned on those skid marks and their absolute positions can be readily determined. Often, however, there is not a full complement of data, and the absolute positions have to be inferred from the available information.

In any event, by using one or more marks to anchor one or both of the cars to the road at the point of maximum engagement it is possible to establish the absolute positions of the vehicles at the point of initial impact.

The preceding assessment outlines how the reconstructionist can answer to the question of where the collision occurred.