The Science of Numbers – Accident Reconstruction

Imagine that you are pulling out of the dealership in your brand new 1936 McLaughlin-Buick and over the hill comes a 1934 Ford V8 Coupe. Despite laying down almost 100 feet of skid marks, the Ford slams into the side of your new baby, taking the shine off it in the blink of an eye.

While the Ford driver did have the right of way, you continue to ponder the length of the skid marks and you are convinced that he was driving much too fast for the stated 30 mph speed limit. However, the only proof you have is a very long set of skid marks and the damage to your vehicle. In the past, the Buick driver probably had very little recourse unless a reliable witness could be found.

The earliest reference I could find to the use of an expert in the field of accident reconstruction was a case heard in New York City in 1940. In that case, the defendant was involved in a motor vehicle accident and the cause of the accident was alleged to be the excessive speed of the defendant’s vehicle. The police measured the length of the skid marks, conducted skid testing at the scene, and sought out the help of a physicist from a local university. At trial, the police reconstructionist was able to take the court through the slide-to-stop formula and prove the vehicle was speeding beyond a reasonable doubt, resulting in a conviction.

Accident reconstruction has evolved dramatically since the early days and the role of the accident reconstructionist has become increasingly important as police, insurance companies, the courts, and the general public seek to answer questions based on the physical evidence that remains after an accident. This example, in which the accident reconstructionist measured the skid marks and calculated the vehicle speed, is still a very common technique and is reliable when skid marks are present at the accident scene. However, much more sophisticated techniques have evolved, not only to determine vehicle speeds, but to answer other questions such as the contribution of the roadway geometry, the visibility of a pedestrian at night, or the injury potential for an occupant in a given crash.

Animated task

Now, computer applications have become commonplace in the analysis of most accidents. Some applications simply allow the accident reconstructionist to apply traditional techniques more efficiently and with greater accuracy. Other programs allow the user to analyze an accident in ways not possible just a few years ago. Computer simulation programs are relatively new to the field of accident reconstruction and they allow the user to reconstruct the accident using the laws of physics and then view the results graphically in the form of a computer animation. (See figures 1 and 2) These simulations differ from early computer animations in that they must adhere to the physics models built into the simulation software. In other words, the movement of the vehicles within the 3D simulation is controlled by the laws of physics rather than the coordinates entered by the program user. The result being that the user is less able to take artistic license in creating the animation.

The physics models used by the simulation programs are generally similar to those used in manual calculations, however, they typically involve a greater number of variables, making the equations difficult to work with in a manual calculation. The computer can calculate in a few seconds what would take a person several days of number crunching. The real power of the computer simulation programs is in their ability to allow the user to quickly test a wide range of variables and thereby test a number of different hypotheses. In addition, a computer simulation can often be used to illustrate the movement of a vehicle in a case where it may be very difficult to convey this information verbally.

Crash test

In addition to these computer applications, the field of accident bio-mechanics has provided a vast amount of research on the interaction between the occupant and the vehicle. The main objective of most of this research has been to improve vehicle safety and reduce the number of serious injuries sustained. However, this data has helped accident reconstructionists to answer such questions as the extent of injury that might be expected if an unrestrained occupant had been wearing the seat belt in a crash. In other cases, it may be possible to determine what caused a particular injury, especially in a multi-vehicle crash. Much of the bio-mechanics research has been conducted using crash dummies, which have evolved to a point where they can closely model the behavior of the human body in a crash.

Another technology which can assist the accident reconstructionist is the event data recorder or “black box” now found in most vehicles. While some have suggested that these devices may eliminate the need for an accident reconstruction, the data downloaded from a data recorder still must be properly interpreted to be useful. One example would be a vehicle that had left the roadway and was involved in a severe side impact. The downloaded data showed the vehicle decelerating to a gentle controlled stop. However, the data recorder only records the vehicle speed in the fore-aft direction and not in the lateral direction. Although the accident vehicle had no forward speed on impact, it was still traveling at considerable speed, albeit sideways.

Evolving field

The manner in which the accident reconstructionist conducts the investigation can be as important as the techniques used. Whether the engineer uses manual calculations or a computer simulation, he or she must be able to demonstrate that scientific principles were followed in the investigation and that their conclusions were reached in an objective manner. In a recent case in which I was involved, it was discovered during the trial that the opposing expert actually completed his report before conducting his field investigation.

In addition, the judge found that the mathematical approach used by the expert to calculate the vehicle speed was a purely theoretical approach that had never been scientifically verified. Based on these findings, the judge ruled the evidence of the opposing expert inadmissible in the trial. The science and technology used in the field of accident reconstruction will continue to evolve as vehicles and roadway systems become more complex.

Although some technologies are currently being developed which in theory would prevent vehicle collisions entirely, these technologies may prove difficult and costly to implement on a large scale.

Accident reconstruction will continue to serve a useful purpose as long as vehicles keep running into each other. Just like the guy in the McLaughlin-Buick, there will always be a need to know what caused or contributed to a motor vehicle accident.