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Vehicular Accident Reconstruction



Motor vehicle accidents are the most common type of incidents producing injury in the U.S. Accident reconstruction is the scientific process of investigating, analyzing, and drawing conclusions about the causes and events during a vehicle collision.  The topic can be of great interest to motorists involved in a collision, especially if criminal charges or litigation are likely and/or if it resulted in severe injury or death.

Reconstructionists are employed to conduct in-depth collision analyses and reconstruction to identify the collision causation and contributing factors in different types of collisions, including the role of the driver(s), vehicle(s), roadway, and the environment. The laws of physics and engineering principles, such as the conservation of linear momentum, work-energy methods, and kinematics, are the bases for these analyses and may make use of software to calculate useful quantities.

The National Highway Traffic Safety Administration (NHTSA) funded the first national guidelines for the standardization training in the field of traffic accident reconstruction in 1985. This led to the establishment of Accreditation Commission for Traffic Accident Reconstruction (ACTAR), an industry accreditation group.  The accident reconstruction provides rigorous analyses that an expert witness can present at trial. Results from accident reconstructions are also useful in developing recommendations for making roads and highways safer, as well as improving safety aspects of motor vehicle designs. These reconstructions are often conducted by forensic engineers, specialized units in law enforcement agencies, or private consultants.

Scene inspections and data recovery involve visiting the scene of the accident and investigating all of the vehicles involved in the collision. Investigations involve collecting evidence such as scene photographs, video of the collision, measurements of the scene, eyewitness testimony, and legal depositions. Additional factors include steering angles, braking, use of lights, turn signals, speed, acceleration, engine rpm, cruise control, and anti-lock brakes. Witnesses are interviewed during accident reconstruction, and physical evidence, such as tire marks, are examined. The length of a skid mark can often allow calculation of the original speed of a vehicle, for example.

Vehicle speed is frequently under-estimated by a driver, so an independent estimate of speed is often essential in accidents. Inspection of the road surface is also vital, especially when traction has been lost due to black ice, diesel fuel contamination, or obstacles such as road debris. Data from an event data recorder also provides valuable information, such as speed of the vehicle a few seconds before a collision.

Vehicular accident reconstruction analysis includes processing data collecting, evaluating possible hypotheses, creating models, recreating accidents, testing, and utilizing software simulations. Like many other technical activities, accident reconstruction has been revolutionized by the use of powerful, inexpensive computers and specialty software. Various types of accident reconstruction software are used regularly by law enforcement personnel and consultants to analyze a collision and to demonstrate what occurred in an accident. Examples of types of software used by accident reconstructionists are CAD (computer aided design) programs, vehicle specification databases, momentum and energy analysis programs, collision simulators, and photogrammetry software.

After the analysis is completed, forensic engineers compile report findings, diagrams, and animations to form their expert testimony and conclusions relating to the accident. Forensic animation typically depicts all or part of an accident sequence in a video format so that non-technical parties, such as juries, can easily understand the expert's opinions regarding that event.

There are two primary types of people that perform accident investigations and/or reconstruction:  the police and independent investigators.  The police will investigate an accident with the intention of determining if any criminal action took place in the accident. Some of the things the police look for are speeding, hours-of-service violations, mechanical violations, alcohol use, drug use, etc.

Independent investigators are normally hired by the insurance companies or attorneys of the parties involved in an accident. The job of the independent investigator is to analyze the causes of the accident so that liability and/or financial responsibility can be determined. These independent investigators are often off-duty police officers, retired police officers, automotive technologists, or engineers. However, investigators from many other backgrounds are also common.

The two main arenas in which accident reconstruction is typically used are criminal and civil litigation. In general, there is little difference between accident reconstruction in criminal and civil cases. The work, time, and money it takes to perform the reconstruction is the same. The big difference between the two venues is the level of detail and the funds available.

In criminal cases, the prosecution usually has little or no budget for accident reconstruction. Therefore, police officers often serve as accident reconstruction expert witnesses. Unfortunately, very few police officers have the training and even fewer have the time to do a thorough accident reconstruction. Individual criminal defendants often fund their defense so there may be little money available for accident reconstruction. The end result is that good quality reconstruction in criminal cases is probably the exception rather than the rule.

In civil cases, plaintiffs’ attorneys are usually willing to spend a reasonable amount of money on quality accident reconstruction, anticipating reimbursement from a winning verdict. On the defense side, insurance companies usually provide funding.  So, again, there is typically sufficient funding for a comprehensive accident reconstruction to be performed when the stakes are high enough.

Accident reconstruction requires a certain level of work, regardless of whether it is for a criminal or civil case and regardless of whether it is a minor fender-bender with low damages or a serious collision with millions of dollars (or serious jail time) at stake.  In fact, minor collisions are often more difficult to reconstruct than major ones, because there is typically much less physical evidence produced in the collision. In a minor collision, the police investigation rarely documents physical evidence. Any physical evidence that is produced by the accident is typically long gone by the time an accident reconstructionist is hired.

To properly perform accident reconstruction, one determines what the physical evidence is and works from there. When appropriate, witness accounts are incorporated into the analysis, but eyewitness statements and testimony are almost always discounted when the physical evidence tells you something else. One must always keep an open mind to make sure that the physical evidence is fully understood and that no evidence has been overlooked.

The main steps in accident reconstruction include 1) obtaining physical evidence; 2) analyzing the physical evidence and witness testimony; 3) forming opinions; 4) generating a report; and 5) testifying at depositions and/or trial.

The first two steps are the most critical. Physical evidence usually includes going to the location where the collision took place and using surveying equipment and cameras to document the roadways, visibility restrictions, signage, and most importantly, any physical evidence still remaining from the collision itself, such as tire marks, gouges in the road surface, and fluid stains from the vehicles.

It is important to note that most vehicles will leave tire marks on the road surface under most conditions during emergency braking, even if equipped with an antilock braking system (ABS). The marks may be faint at times and may or may not be documented, but they are often there.

If an accident reconstructionist is hired soon enough after the actual event, much of the physical evidence left by the collision may still be at the site. However, even if the physical evidence is largely gone, there are techniques to reacquire the data if police investigators or others take sufficient photographs shortly after the accident.

Proper photographic techniques are time-consuming and involve using special cameras and/or computer software, but are essential in reconstructing the collision in the absence of key physical evidence.

There are a number of factors that are often analyzed in accident reconstruction, including nighttime visibility, visual conspicuity, lamp filament analysis (to determine if vehicle headlights were on at the time of collision), traffic light timing and sequence, roadway friction issues, hydroplaning, mechanical failure, signage, braking performance, and yaw mark (sideslip) analysis, just to name a few. Every collision presents a different set of analysis requirements for accident reconstruction.


A more recent class of physical evidence involves event data recorders (EDR), often referred to as “black boxes.”  Although the latter term is a bit of a misnomer, more and more vehicles are equipped with a means of recording crash information that is useful in determining what happened. At the very least, typical recording equipment will register information regarding the crash pulse, which is the change in speed of the vehicle during the collision.

Some EDRs will provide up to several minutes of data prior to the collision, including vehicle speed, brake light and anti-lock brake status, transmission gear selector position, throttle position, steering wheel angle, vehicle rotation rates, seatbelt usage, and more.

It is critically important to know how to interpret the EDR data. One of the biggest effects that such data have had is keeping everyone “honest,” since the collected information often provides a solid foundation that cannot be ignored. Hence, the “black box” does not replace accident reconstruction but can actually enhance it.

Data from EDRs can be very valuable evidence.  However, this data is not a substitute for a crash reconstruction, but only used as evidence in a crash reconstruction analysis.  A crash reconstruction may include any or all of the following: (1) the analysis of vehicle movements, (2) the analysis of vehicle speeds, (3) the analysis of vehicle impact angles, (4) the analysis of time-distance relationships of vehicles, etc.

It should be stressed that EDR data cannot stand alone and should only be used as a tool to analyze a crash, just as any other evidence such as skidmarks and vehicle damage would be used.  We often find cavalier investigators presenting this data at face value with no understanding of where the data is coming from or how the crash dynamics can affect it.  However, the data must be analyzed in conjunction with other evidence to ensure its reliability.

A reconstructionist must properly analyze and have an understanding of what the vehicle dynamics were prior to the collision.  Then he must determine whether the vehicle's pre-crash dynamics affected the recorded vehicle speed.  He must also know that the vehicle speed data comes from a vehicle speed sensor (VSS) that effectively measures the rotation of the wheels and that wheel speed and vehicle speed are not always the same.  For example, a vehicle that is rolling over with its tires off the ground, or sliding sideways, or braking hard, will have a wheel speed that is different than its forward speed.  These factors have to be taken into consideration to correctly analyze the data.

On a side note, whether the law says the owner of the vehicle “owns” the data appears to be largely irrelevant.  It’s hard to find a case where access to the data was not ultimately agreed to by the owner or ordered by the court.

Accident reconstruction can ordinarily determine what occurred in an incident without the availability of EDR data.  By using electronic surveying equipment, a computer-generated scale diagram can be produced and may be used to reconstruct the accident.

Analysis of the physical evidence varies from collision to collision. It is almost always necessary to study the physical evidence to determine how vehicles moved prior to, during, and after impact. Estimating vehicle speed is also important and typically requires application of the physics of conservation of momentum and conservation of energy.

Sometimes data from the vehicle are nearly self-explanatory relative to speed, and not much else needs to be analyzed. There are also occasions where it is necessary to accurately measure how much a vehicle was crushed (damaged) during an accident.

That information can then be used in conjunction with government or other crash test data to calculate the energy of the collision itself. Through this process, a complete picture of the vehicle speeds before and after the collision can be developed.

In addition, it is often necessary to perform a time and motion analysis to determine, (1) the relative positions of the vehicles through the course of the accident, (2) what each driver was able to see, and (3) how much time they had to perceive and react to the situation.

Once all the evidence has been gathered and interpreted, it is time to formulate opinions as to what happened in the collision. Driver perception-reaction should be accounted for, and what each person did or failed to do requires analysis to reach conclusions on the ultimate cause of the collision.

Once the accident reconstructionist has formed opinions, it is typically necessary to prepare a report containing conclusions. Many times the accident reconstruction report is used by other experts – such as the biomechanics specialist, who needs to know the speeds and acceleration rates in a collision to determine cause of injury.

Finally, accident reconstruction specialists are often asked to provide expert testimony regarding the details of their reconstruction. This is usually done by deposition, or if the case goes to trial, courtroom testimony. In a deposition or trial, the accident reconstruction expert is typically asked how the analysis was conducted and what foundation exists for the opinions rendered.

Twenty years ago, most civil cases probably proceeded until all accident reconstruction depositions were taken. Then, 95 percent of the time, cases would be settled before trial. Today, more and more cases are settled even before reports are written or depositions taken. One way or another, most cases involving accident reconstruction settle prior to trial.


There are many types of accident scenarios (i.e., head-on, rear-end, right-angle, or roll-over), each requiring a different method to reconstruct. However, generally speaking, an accident reconstruction will determine how much of a vehicle’s speed is lost in each part of a collision sequence.  For example, let’s say a vehicle skids off the road and strikes a pole. A reconstruction of this accident would use the vehicle’s damage profile to calculate the speed at which the vehicle struck the pole and combine that with the amount of speed lost while the vehicle was skidding, to determine the speed of the vehicle before the driver began to brake.


A little more complex accident reconstruction involves two vehicles first skidding, then colliding with each other, then sliding to a stop. For this type of accident, the method used is called the Conservation of Linear Momentum, which takes into account the weights of the vehicles, the angles at which they collided, and the places where they came to rest.

For example, let’s say there is a two-vehicle collision where one vehicle is traveling south and the other is traveling west. After these two vehicles collide, the rules of physics dictate they will move generally southwest, with all of the southward momentum resulting from the southbound vehicle and all of the westward momentum resulting from the west-bound vehicle.  If provided with the weight of each vehicle, how far each vehicle moved south, and how far each vehicle moved west, the reconstructionist can calculate a collision speed for each of the vehicles. This collision speed for each vehicle can then be combined with its speed loss from pre-collision skidding to calculate its pre-braking speed.


Once pre-braking information has been attained, the pre-braking speeds can be used to both look at time-distance relationships before collision and to pose “what-if” questions. For example, if one of the vehicles was found to be speeding before collision, the reconstructionist could pose the question what would have happen had that vehicle not been speeding. By moving that vehicle at its calculated speed back from the initial braking point by, say, 1.5 seconds for a normal driver perception-reaction time, the reconstructionist can determine a point of perception. Then by asking what if that driver had been traveling at the speed limit, had taken 1.5 seconds to perceive and react, and had locked the brakes, the reconstructionist can determine if the accident could have been avoided if the driver had simply obeyed the speed limit.


The application of crash reconstruction principles are very important part of analyzing motor vehicle failures.  The interpretation and analysis of physical evidence to determine what the vehicles did during a crash sequence is one of the basic parts of crash reconstruction. When analyzing mechanical failures, knowing how to interpret the physical evidence and determine if it is consistent with a mechanical failure is very important.  Equally important is a detailed understanding of the design, construction, service, and maintenance of motor vehicles.  Additionally, appropriate experience and skill is needed to disassemble vehicle components for inspection without causing any damage.

A failure analysis should accomplish two primary goals.  The first goal is to determine if a component found to be broken did or did not cause the crash.  This part often relies heavily on roadway evidence and the application of reconstruction principles.  For example, in a very simple case where a wheel is alleged to have come off a vehicle, an inspection of the roadway evidence may show very clearly that the corresponding tire left a skid mark leading into a collision with another vehicle.  Under these circumstances, a conclusion that the wheel was on the vehicle