(Published in Subrogator Magazine, Fall 2007)


Charles C. Roberts, Jr., Ph.D., P.E.



In the “old days,” automobile accident reconstruction testimony was often offered with little scientific or computational basis. Controlled (expert) witnesses would offer estimates of impact speed based on, for example, years of experience as a police officer attending to actual automobile accidents.  With the advent of more scientific approaches where calculations are rooted in laws of physics, accident reconstruction has evolved into a more scientific methodology utilizing computers for calculations (Ref 1) and instruments for testing automotive vehicles. Still, some jurisdictions gave more weight to eyewitness testimony speed estimates of a vehicle than to speeds determined by scientific means (Ref 2).  An eyewitness may be able to testify that a vehicle was traveling in a certain direction, but is unlikely to offer accurate testimony as to the speed of the vehicle.  Over time, the courts have come to realize that scientific means have evolved in accuracy and can help the trier of fact. Consequently, accident reconstruction analysis, which is scientifically based, is now typically allowed in court, despite the existence of eyewitness testimony. This article reviews what has changed and what remains the same in accident reconstruction over the years.

Accident Scene Investigation

Classically, accident scenes have been documented by film cameras and tape measurements. The accident reconstructionist walks the scene, takes measurements and documents on paper. With the evolution of inexpensive digital technology, digital cameras have displaced the film camera, and electronic theodolite (transit) measuring is beginning to displace manual measurements.  For example, Figure 1 is a diagram of an accident scene measured by an electronic transit, generated from total station software. Total station is a measuring device used by land surveyors, but is now being used by police departments and investigators to document accident scenes. The instrument uses a light beam to measure objects at the scene and converts this information to x, y and z coordinates. The data is then downloaded to a CAD program for scene drawings and calculation purposes. There are over 100 data points measured with the total station to generate the scene diagram of Figure 1.


Figure 1

Figure 2

Figure 2 shows a drawing of a typical theodolite with microcomputer measuring device and optical viewing scope. The operator typically has a hand held device to download data from the theodolite, to be used in the analysis. The advantage of the new technology is more accuracy, more data points and less measurement time.


Figure 3

Another aid in analyzing the accident scene is the availability of relatively high resolution satellite imaging, as shown by the satellite photo of Figure 3. Resolutions on the order of one foot can be purchased for many areas in the U.S. When total station data is not available, these images can be scaled to yield measurements as well as overall views of the accident scene. They are also valuable in explaining the features of the accident scene in court.

Vehicle Inspection and Analysis

Vehicle damage pattern analysis has always been an important aspect of accident construction and will continue to be utilized in the future. The damage patterns help determine orientation of vehicles at impact and severity of the impact.  Some new technology that is helping the reconstructionist, is the onboard data recording technology available now in many new vehicles.  This includes the sensing diagnostic module for air bag deployment (black box) as well as onboard trouble records in vehicle memory. Figure 4 shows a typical black box (they are actually silver), indicated by the arrow, being downloaded to a lap top computer. 


Figure 4

In this instance, the black box was removed and connected directly to the computer because of severe damage to the main vehicle electronic bus system. Many times, the data can be extracted without removing the black box.  Data extracted from the black box includes vehicle speed, seat belt usage and velocity change: a measure of the severity of the accident. See Reference 3 for an article detailing black box technology. With the advent of anti-lock brakes on vehicles, tire skid marks are less apparent, since the antilock brake system is designed to reduce skidding. It is becoming more difficult for analysts to measure these faint tire marks especially several days after the accident. The black box speed data has evolved at the right time to offset this loss of data.


Other onboard data is available through more advanced diagnostic tools (Figure 5) that store fault codes on various processors throughout the vehicle. The tool is connected to the DBR2 port on the vehicle and downloads information stored onboard the vehicle. This information can give insight into the mechanical and electrical condition of the vehicle and possibly signal the existence of a defective condition in the vehicle that could have caused an accident.  

Figure 5

Figure 6 is an example of another electronic testing device, a brake performance analyzer. Mounted on the dash, the vehicle is tested to determine if braking performance of a vehicle is adequate.  Data read-out is in G’s of deceleration, or drag factor.


Figure 6



The Reconstruction

Hand calculations using equations derived from scientific physical laws are still used in reconstruction. However the proliferation of inexpensive computer technology has spawned a variety of computer programs used




Figure 7a                            Figure 7b



Figure 7c


to calculate impact speeds and provide simulations of the accident.  Figure 7a shows impact positions of two vehicles involved in an accident.  Figure 7b shows the rest positions of the vehicles, determined by scene data. The hypothesized impact and speed conditions are input into the computer, and the results reviewed (Figure 7b). If the rest positions from the calculation match the positions documented at the scene, then the input data is most likely the impact speed and orientation of the vehicles.  Figure 7c shows vehicle crush or damage calculated by the program. This is compared to the actual damage to the vehicle as a measure of the accuracy of the calculations. The program shown above uses the SMAC (Simulation Model of Automotive Collision) computer analysis developed by the National Highway Traffic Safety Administration. Simulations can then generate animations. These are short videos of how the accident occurred, which are good demonstrative evidence for a jury. The computer programs utilize the same basic laws of physics as do the hand calculations but are more rapid and many times more accurate. It should be noted that the computer is not offering an opinion as to speed or impact orientation. It is the reconstructionist who offers the opinion based on scientific data and analysis tools (computer) commonly used in the scientific community. 


Accident reconstruction has evolved and is adapting to new technologies. The once prolific skid mark has been slowly disappearing because of anti-lock brakes, which leave little or no tire mark.  Black box data showing pre-impact speed has replaced some of this information as software becomes available to access this information. Despite all these new and improved tools that are available,  the reconstrutionist is still the ultimate formulator of opinions offered in court as to how an accident occurred.  Other articles that concern accident reconstruction and may be of interest are outlined in References 4-8.


1.    "Computerized Accident Reconstruction," Insurance Adjuster Magazine, June 1983, p50f.

2.    Colonial Trust and Savings Bank v. Kasmar, et al. Third District No-3-89-0034, Illinois.

3.    “Black Box Aids in Subrogation,” Subrogator, Spring/Summer 2004, p104f.

4.    "Automotive Lamp Examination," Insurance Adjuster Magazine, February 1984, p42f.

5.    "Response Time," Claims Magazine, June 1998, p30f.

6.    "The Anti-Lock Effect," Claims Magazine, March 1994, p69f.

7.    "Damage Patterns on Vehicles Reveal Much for Accident Reconstruction," Claims Magazine, June 1996, p36f.

8.    "Truck Accidents - Investigating Probable Cause," Insurance Adjuster Magazine, October 1985, p43f.