Daniel N. Olson, M.S.M.E., P.E.
The Accident
Take the situation where your insured just told you about an accident with a fatality that happened last week. It may take an additional two weeks to collect more information. The information you have is in two crashed vehicles, three polaroids taken shortly after the accident by an untrained but interested person, and in one surviving driver who doesn't want to talk. The answer to the question "where the collision occurred and who is responsible" is not readily apparent.
You decide to consult with an engineer to reconstruct what happened. Do you have enough information for the engineer to start the investigation?
The Reconstruction
At first it looks bleak. Well, don't give up. Look at Figure 1. Although the quality of the photograph is poor, it still preserves valuable information for the reconstructionist. Arrows 1 and 2 point to skid marks that have since worn away. Arrow 3 points to a traffic groove that is still visible today. Sand was spread on spilled oil and a stain from the oil still remains.
The skid marks are definitely good clues to show the path of one of the vehicles and help determine where the collision occurred. The problem was figuring out how to accurately plot the skid marks on a precise map of the scene. As it turns out there are other clues in the photograph.
Since a straight line in a projected view is a straight line on a plan view, the reconstructionist projected the edge lines of the sidewalk into the photograph of the road and onto the plotted map of the scene. This provided additional references to help fix the location of the skid marks and therefore the path of one of the vehicles. A similar analysis of another polaroid helped to locate the skid marks of the other vehicle. Now with the path of both vehicles plotted, the precise location of the collision could be determined.
More Is Better
Although we were able to reconstruct the accident with just a few pieces of information, more is always better. Here's a list of items that can provide useful clues to the cause of the accident.