An Accident Reconstruction Primer


Traffic accidents are the elephant in the room nobody wants to talk about. We go about our daily lives without thinking for one moment about what might happen if we are involved in a serious crash, because we always believe that our vehicle will not be the one that is being craned out of a deep ravine, or loaded onto a flatbed truck in a heap of twisted metal. Unfortunately, statistics suggest it is more likely than not that some who read this will be involved in a serious accident at some time in their life. Further, in our increasingly litigious culture, there is exists a very real potential that we will become involved as plaintiffs or defendants in a civil or criminal action resulting from a car accident. When this happens, you should hire the best attorney you can afford, and preserve as much evidence as you can. Every driver should understand a little about what makes up a typical accident study, if only to protect his or her own interests in the aftermath of a terrible collision.

Generally speaking, accident reconstructionists are professional experts and consultants who are usually retained by legal advocates. We are oftentimes engineers, physicists, ex-police officers, or highway safety professionals, but we are specialized by experience and training in the analysis of traffic collisions. Though we usually work for the legal advocates defending or prosecuting a particular case, we are independent of any particular “side” in a legal action. The story of each accident study is different, but the efforts accident reconstructionists travel along well-worn paths to eventually arrive at our expert opinions.

At the first consultation with a potential client we may reject a case because it does not appear that we can offer any help (e.g. their client is clearly at fault, there is no legitimate accident to reconstruct, the deadline is too soon, etc.). Sometimes initial contact is made at the accident location so that the issues and circumstances can be better understood and an informed decision can be made as to whether or not we should become involved. Accident reconstructionists try to avoid being compelled to render opinions contrary to our client’s case.

Determine the facts. Usually the first evidentiary item found in a typical file is the traffic collision report (TCR), as they are called in California; elsewhere they have other names. This documents the police investigation into the accident and usually provides most of the factual information necessary to conduct an analysis. Although some accidents are more thoroughly investigated by law enforcement than others, most reports identify the parties and their vehicles, provide witness statements and contact information, offer some description of the accident scene as it was found by the investigating officer, and may contain some measurements and/or diagrams that describe the locations of various items of physical evidence. This evidence might include any tire marks, gouges, dirt tracks, debris, and the vehicles’ points of rest. Hopefully the investigation includes photographs of the scene, which are critical to any traffic collision report. It seems that police officers do not always think so because only about 50% of the crashes I investigate include scene photographs, and less than half of those cases include useful photographs because of the low priority most agencies put on accident investigation training. This is a terrible shame because peoples’ memories fade with time, but photographs can preserve evidence that was not collected at the scene of the crash or memorialize temporary conditions that no longer exist.

If you are ever involved in a traffic accident, the best advice I can offer is that you do not provide a statement to the police at the scene. This may seem a little strange coming from a person who relies heavily on the information presented in the police report, but it is in your own best interest to provide a written statement at a later time. Immediately following an accident, emotions are high and a person might say inaccurate things because they feel guilty or feel bad for another party who was hurt. I have never read a statement that did anything to help the person making it, but I have read hundreds that led to their arrest. I suggest a written statement because police officers are humans who cannot be expected to paraphrase your statement perfectly. Any differences between your statement and what eventually is recorded in the report can completely change the meaning of what you said. Once the officer has filed his report, the statement cannot be taken back so choose your words very carefully.

Site Investigation. After we have the location and some details regarding the issues we have been asked to evaluate, we visit the accident site to conduct an independent topographic survey. We conduct a survey as it would be conducted by a traditional surveying firm with the obvious exception that we pay particularly close attention to any remaining physical evidence. The site is photographed comprehensively and the survey might also include surface friction tests, sight-distance measurements, video, radar speed surveys, illumination level readings, ball-bank measurements, and any other tests requested by the client or that we feel will help us in our investigation. Sometimes when the accident site is in another state or if we are severely limited by time, other surveying firms are contracted to provide us with topographic measurements. However, most accident reconstructionists prefer to conduct their own investigation at the accident site.

Vehicle Inspection. If the subject vehicles have not been destroyed or repaired, a reconstructionist will inspect them to collect evidence that is helpful in most studies and critical in product liability lawsuits. Inspections can occur at any time as the vehicles become available. My personal preference is to see the cars before the field review because sometimes questions arise during the vehicle inspections that can be answered by collecting evidence at the site. Without the clues sometimes discovered while examining the vehicles, I might miss the importance of something unusual or unique at the accident location.

Inspecting the vehicles can provide clues to the severity of the impact, the occupant motion, the crashworthiness of the vehicle, seatbelt usage, airbag performance, and may implicate the vehicle itself in the cause of the accident (e.g. tire puncture/blowout, electrical fire, brake failure). Most importantly, this information can sometimes be preserved by the owner of the vehicle through photographs. Any time you are involved in a traffic accident, you should take as many photographs of your vehicle as you possibly can before it is destroyed or sold for parts. The value of these photographs cannot be overstated, and one should not rely on 6, 8, or even 12 photographs to tell the story. Film is cheap and digital prints are even cheaper, so if you are physically able to do so, do not hesitate to take one-hundred or more photographs of your crashed vehicle.

A majority of vehicle inspections concern the recording of certain measurements that will later allow us to estimate the energy that was dissipated by the crushing of metal and deformation of structure that occurred at the time of an accident. Most people intuitively understand that the greater the level of permanent displacement to a vehicle’s form, the greater the effort must have been to displace it. In strictly Newtonian terms, if we consider crash-related damage to be the result of work that is done at the time of the accident, and that the energy to perform the work is provided by the mass and speeds of the vehicles in the collision, we can develop formulas to estimate the amount of energy that was required to do the work. If we can determine a reliable estimate of this work energy, we can draw conclusions about the speed change that a particular vehicle may have experienced as a result of the damaging event. This might ultimately lead to some conclusions regarding the approach speeds of the vehicles.

Discovery. For a period of time, which varies by statute and the jurisdiction of the case, the legal minds conduct discovery and provide us with other forms of investigative information. The discovery material often includes depositions of percipient witnesses to supplement statements from the TCR, depositions of the appropriate highway officials that have knowledge about the history and design of the roadway, depositions of other experts retained by various plaintiffs and defendants, photographs of anything related to the crash, and basically any information not part of the initial police investigation. Discovery can sometimes drag on for years during which information trickles into our office in bits and pieces. We continue working during discovery and revise our opinions and conclusions, if necessary, as new information is received. Our investigatory services are considered part of the discovery process because the reports and opinions we produce for our clients, with certain exceptions, become part of the public record and must be provided to any party who asks for them.

Accident Reconstruction. Once we have gathered sufficient discovery, identified what we are to evaluate, and have been given authorization, we can finally proceed with the accident reconstruction. Usually this begins with developing a diagram of the accident site based upon our survey, the TCR, and any photographs of the scene taken at or near the time of the accident. When I first started in the field, before micro-computers were commonplace, I used to draft everything by hand with technical ink pens. That was quite time consuming and with advances in technology I am happy to say most no longer draft using technical pens and lettering guides. I do all of my diagram development using AutoCAD and various third-party add-ons to the program that have greatly reduced the time required to put a diagram of the roadway environment together.

Once there is a diagram, the accident can be plotted using the discovery information. This is where the police investigation becomes critical and its shortcomings and strengths will become apparent. Since it is nearly unheard of for a reconstructionist to visit the scene of an accident, we must rely heavily on the police investigation for accident scene data. If we visit the site within a short time of the accident, we can record the locations of evidence independently. When accidents occur on high speed expressways, it is oftentimes impossible to get this data without shutting down traffic lanes, which almost never happens. I cannot express how important the police investigation is to our work, nor how often it is not taken seriously by the investigating agency. It is hard to believe by the uninitiated, but an untrained or unmotivated officer is not held responsible for any errors, mistakes, or flat out wrong information that may be in a TCR unless it can be proven that it was intentional. Sadly, careless or slanted accident reporting is simply accepted as the status-quo by most police agencies. This is not to say that I do not see very good, well documented, well researched, and excellent police investigations; but it is unfortunate to accident victims that conscientious police reporting is by far the exception rather than the rule.

Our accident plot usually includes scale representations of the vehicles in their positions at the point(s) of impact, points of rest, and any physical evidence associated with the accident superimposed on the scale diagram. This allows us to measure specific items like the distances traveled before and after impact, the approach and departure angles of the vehicles, the deceleration forces that influence a vehicle (skids, gouges, etc.), and any other relevant facts with respect to the physics of the crash. This step is of critical importance because it provides the basis for everything that follows. To wit, it is very difficult to evaluate the deceleration of a spinning vehicle without a good accident plot because the forces compelling that vehicle to slow are transient and are a function of what is known as the slip angle. The only way to know the slip angle at a given point in time or space is to know where the vehicle came from, where it is going, and where it is now, based upon the physical evidence. All of the information obtained during discovery has its appropriate value as it is examined within the context of a traffic accident, but how much value it has requires some subjective analysis on the part of the reconstructionist.

Calculations. Usually we will be asked to determine vehicle speeds in a given accident, but this is not always possible from the data provided. There are various methods we employ to make this determination. Some methods are empirical, meaning they are based upon comparing the subject accident to physical models that have been previously developed by controlled testing. This is how we usually evaluate damage energy, vehicle deceleration when we cannot test the actual surface friction, and other points that we have been asked to address. Other methods are theoretical, which is to say they are based upon the theories and principles of physics and mathematics, rather than documented observations from testing. That is not to say that theoretical methods have not been tested, but that they were not developed from testing. In accident reconstruction, non-empirical methods are derived from Newton’s Laws of Physics and from his formulas of uniform motion. Higher order math is employed to be sure; however, nearly all but the most esoteric theoretical analyses can be boiled down to Sir Isaac Newton’s observations of our physical world. To simplify the concept, empirical methods employ inductive reasoning and theoretical methods employ deductive reasoning, but they both can help us understand the physics of accident reconstruction.

Some of the methods that are typically used to estimate speed include the theory of the conservation of momentum, conservation of energy, damage energy estimates using crash test data from the National Highway Transportation Safety Administration (NHTSA), and the myriad forms and permutations of Newton’s particle kinematic equations (I have even derived a few new theoretical formulas using these equations that are useful in certain situations). Sometimes, due to a lack of the appropriate data, there is no other way to estimate a vehicle’s speed in a crash than to consider the witness statements in the police report, or review the available deposition transcripts of percipient witnesses, and arrive at an estimate based upon the entire pool of knowledge one develops only through experience in crash investigation.

Sometimes it is not important to calculate the speed of a vehicle at all because it has been stipulated to, or because it is irrelevant to the immediate question at hand. It might be that what we really want to know is the time elapsed between events, or the distance a vehicle was from impact at the point in time that another party looked to see if it was safe to proceed. On other occasions, usually for highway design evaluations, we are more concerned with the operational speed or design speed of a roadway, so that we can evaluate the roadway using more objective criteria before comparing it to the subject accident. For example, if a roadway is designed for 65 mph traffic and we calculate that at 65 mph a car will loose traction around a curve on that roadway, then the curve should be engineered to reduce the high potential for run-off-road and head-on accidents at the deficient curve (e.g. curve specific signage, reduced speed limit + enforcement, increased illumination, advanced warning, etc.). In this example we do not need to know what the speed of a subject vehicle was to evaluate the potential for safety problems at such a location. Whether or not an analysis like the example is relevant to a particular accident situation is another matter altogether and that question is sometimes answered only through a synthesis of concepts from the fields of accident reconstruction, law, psychology, kinesiology, automotive engineering, highway engineering, and logic.

Conclusions. Now that everything has been brought together and thoroughly examined, we present our findings and recapitulate the facts that are most important to our analysis. This is probably where most reconstructionists will have disagreements between them. Depending on the hierarchy of facts, two analyses of the same accident may have different conclusions. If more weight is given to witness testimony and less to the police measurements, one conclusion may be presented. If the opposite is considered, another conclusion can be opined. It is not uncommon for the ultimate hierarchy of facts to be determined by a judge, legal referee, or the people asked to sit on a jury in a court of law. Few of these individuals are scientists, and even fewer are accident reconstructionists or highway engineers. They rely on the opinions and conclusions presented by experts on both sides of a case and must weigh which represents a more reasonable reconstruction of the events in a traffic accident. The presentation of opinions is therefore very important to an accident study, and the best scientific analysis by the research teams at JPL is worthless if it is not understood or believed by the people who must ultimately decide on its validity.

Sometimes the best analysis is not always the most accurate because automobile accidents rarely require extreme precision. Conclusions are often presented that consider ranges of speeds or distances that make very precise calculations impractical. For example, it makes little difference to most people if a vehicle is traveling at 54.2 mph or 56.1 mph, because they would say that car is going 55 miles per hour. Furthermore, I cannot think of a case where such a fine distinction could be discerned by the person in the car (or anyone else) at the time of the accident so what relevance could it possibly have on its cause? The obvious conclusion is that precision is important, but clarity and simplicity have their place in determining the hierarchy of facts that lead to a valid reconstruction.

Sadly, it should be mentioned that there are reconstructionists and engineers, driven by notoriety or money, that sacrifice their scientific integrity to achieve selfish ends. Sometimes reconstructionists are victim to the otherwise altruistic desire to do the best job possible for their clients, and can become blinded to the true hierarchy of facts. Other times, inaccurate or disingenuous conclusions are presented by experts who would rather not be bothered by the truth when it conflicts with the claims made by their “side” in a legal action. At one point or another every expert is presented with a difficult situation, and it is the character of the individual reconstructionist or engineer that determines how they will respond to it. As an individual accident reconstructionist, and as part of Krueper Engineering and Associates, Inc., I try to always be conscious of the greater responsibility consultants have as human beings and as trusted experts to present opinions and conclusions that we believe to be grounded in a pursuit of the truth, and by extension, the pursuit of safer ground transportation. Sometimes the truth is that a person has, whether through the actions or inactions of people or governments, been damaged in a crash; and they should be appropriately compensated. Other times we may conclude that a party was driving without appropriate care and is responsible for their injuries (and perhaps others’ injuries as well). Rarely is it so black and white because frequently the truth lies somewhere in between the claims made by both sides in a legal conflict. Most accidents are the result of many disperse and seemingly unrelated circumstances that converge at a particular time, in a particular place, and ultimately result in a serious traffic collision.

This is what accident reconstructionists and highway safety engineers do. We pull all of the information and data from every source available to determine what the circumstances were, why they came together as they did, and how they are relevant to the cause of a crash. Most importantly, an opinion is almost always offered that describes ways that the accident might have been avoided, or made less severe for the injured parties. As professionals, most reconstructionsists feel that it is a matter of no small importance that we make positive efforts to improve our collective driving experience. Whether through the advocacy of improved methods of roadway design and construction, or by holding a company, or government entity, or careless driver financially responsible for their level of causation in a traffic accident, we hope to make vehicular travel safer with each accident reconstruction and investigation. It is not likely, or even possible, the investigation of traffic accidents might someday eliminate traffic accident injuries and fatalities. However, I believe that the majority of accident reconstruction professionals would consider their lives a success, and gladly look for new employment, if our services were no longer necessary because traffic accidents had become anachronisms of a bygone age.