PHYSICAL EVIDENCE IN MOTOR VEHICLE COLLISIONS
It may be surprising for us to discover that we use physical evidence every day of our lives to learn about our environment and to complete even the simplest tasks.
Consider the simple task of walking on grass. Although we are not conscious of it, we select the style of our gait according to the quality of the grass we walk on. Through past experience we appreciate qualities such as wetness or ruggedness and we adjust our movements accordingly. If we did not attend to it we would continually fall on our faces.
Typically, we are only conscious of the minimum information needed to safely complete our walking task, but when we observe our surrounding environment, other information becomes obvious. We may find trails of flattened or missing grass which tells us other traffic has passed this way. If we look closer, we can find a myriad of deposits which tell us the history of happenings in the area. Consider that cigarette butts, candy wrappers and similar deposits have their story; if we take the time to study them.
Consider the grass. Its height can tell us whether it is cut frequently. If we get down on our hands and knees and examine the cut blades of grass we could estimate when the cutting occurred. The blades may be resting on the surface or buried under new growth.
Normally, when we walk, we do not think of this sort of detail. It is necessary; and typically, we are thinking of totally different needs. But, if it became necessary and we spent a considerable amount of time studying these details, we would become experts at identifying information which we were incapable of doing previously. The layering of cut grass blades would become so familiar to us that we would become better and better at predicting when it was cut.
In a way, this basic example from our everyday lives provides some insight into how physical evidence is used in motor vehicle accident reconstruction.
When we do not examine physical evidence on a frequent basis we are similar to the average grass walker. Sometimes, we are unaware of the evidence we are looking at while at other times, we do not take the time to look closely enough to discover that there is evidence there.
In contrast, the Accident Reconstruction expert has spent considerable time examining the very small details of a collision. He encounters evidence he has seen before, therefore he is keenly aware of it's difference. When his job is done correctly there is no replacement for its value.
For example, seatbelt use is a major concern to most investigators. It is unlikely that you can obtain correct information from an interview with an unrestrained driver in a collision. Yet, with the proper examination of the physical evidence, seatbelt use can always be determined in collisions where it is an important issue. By obtaining an accurate description of the injuries, by examining the vehicle interior for occupant contacts, and by making a thorough examination of the restraining system, an accurate opinion can always be provided. The problem is that often, most of this evidence is not collected.
In another example, physical evidence should be used to accompany speed calculations. Physical evidence can be used to make a precise assessment of the point of impact (P.O.I.) and final rest positions (F.R.Ps.). It often identifies how the vehicles were approaching the P.O.I. and in what manner they moved from the P.O.I. to their F.R.Ps.
In other situations, the vehicle crush is important. Its magnitude can provide an estimation of the collision severity. The general location of the crush can tell the investigator that certain hypotheses about how the collision occurred and impossible.
The deformed shape of the vehicle will tell the expert the direction of the force causing the deformation. It is important to note that, although each individual collision is unique, many collisions are similar. They result in similar damage patterns which can be compared. This comparison is invaluable in identifying how the collision occurred.
Finally, the vehicle surfaces can be examined for specific points of mutual contact which allow the investigator to identify the exact overlap of one vehicle with respect to another. For example, the license plate on one vehicle will produce an imprint or transfer on the surface of the other vehicle. By identifying the location of these "mutually contacting parts" the reconstructionist can manoeuvre scale diagrams of the vehicles to provide an accurate portrayal in inter-vehicle movement.
In summary, we should not be surprised that experts provide opinions about events with seemingly very little evidence before them. At times, it may appear that the opinions are drawn out of a hat. But, most often we forget that the expert sees evidence that we are not aware exists. The opinion he gives is based not only on the accident at issue, but on its relationship to the numerous similar events he has investigated in the past.
So, don't be alarmed if you are walking in the grass and see some person on all fours with their nose in the grass. They either fell on their face or they are an expert.
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